Add common configuration files for linters and other programs

.clang-format

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Basic clang-format config for this project.
+# See https://clang.llvm.org/docs/ClangFormatStyleOptions.html for more info.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+---
+Language: Cpp
+BasedOnStyle: Google
+ColumnLimit: 80
+AlignAfterOpenBracket: AlwaysBreak
+IncludeCategories:
+  - Regex:           '^<.*>$'
+    Priority:        1
+  - Regex:           '^"gtest/.*"$'
+    Priority:        2
+  - Regex:           '^".*"$'
+    Priority:        3
+IncludeIsMainRegex: '([.](test|benchmark))?$'
+---
+Language: Proto
+BasedOnStyle: Google
+...

.editorconfig

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Common editor configurations for this project.
+#
+# EditorConfig is a file format for specifying some common style parameters.
+# Many IDEs & editors read .editorconfig files, either natively or via plugins.
+# We mostly follow Google's style guides (https://google.github.io/styleguide/)
+# with only a few deviations for line length and indentation in some files.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+root = true
+
+# IMPORTANT: some of the other config files (.markdownlintrc, .jsonlintrc.yaml,
+# etc.) also have settings for indent and line length. When making changes to
+# this file, make sure to update the other files to match.
+
+[*]
+charset = utf-8
+indent_style = space
+insert_final_newline = true
+spelling_language = en-US
+trim_trailing_whitespace = true
+max_line_length = 80
+
+[{BUILD,*.BUILD,*.bzl,*.bazel,.bazelrc}]
+indent_size = 4
+
+[{*.cc,*.h}]
+indent_size = 2
+
+[*.json]
+indent_size = 2
+
+[*.py]
+indent_size = 4
+
+[*.sh]
+indent_size = 2
+
+[{*.yaml,*.yml}]
+indent_size = 2

.hadolint.yaml

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Hadolint (a Dockerfile linter) used in this project's CI checks.
+# Info about options can be found at https://github.com/hadolint/hadolint/.
+# Rule/error numbers are described at https://github.com/hadolint/hadolint/wiki
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+format: tty
+no-color: false
+no-fail: false
+failure-threshold: error
+ignored:
+  # Multiple consecutive RUN stmts are usually deliberate. Don't flag that.
+  - DL3059

.jsonlintrc.yaml

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Project configuration for jsonlint (https://github.com/prantlf/jsonlint).
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+comments: false
+compact: true
+continue: true
+endOfLine: lf
+indent: 2
+log-files: false
+no-duplicate-keys: true
+patterns:
+  - '**/*.json'
+  - '!**/node_modules'
+singleQuote: false
+trailing-commas: false

.markdownlintrc

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+{ // -*- jsonc -*-
+  // Copyright 2025 Google LLC
+  //
+  // Licensed under the Apache License, Version 2.0 (the "License");
+  // you may not use this file except in compliance with the License.
+  // You may obtain a copy of the License at
+  //
+  //     https://www.apache.org/licenses/LICENSE-2.0
+  //
+  // Unless required by applicable law or agreed to in writing, software
+  // distributed under the License is distributed on an "AS IS" BASIS,
+  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  // See the License for the specific language governing permissions and
+  // limitations under the License.
+
+  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // Markdownlint linter configuration for this project.
+  //
+  // Note: there are multiple programs programs named "markdownlint". We use
+  // https://github.com/igorshubovych/markdownlint-cli/, which is the one you
+  // get with "brew install markdownlint" on MacOS.
+  //
+  // These settings try to stay close to the Google Markdown Style as
+  // described at https://google.github.io/styleguide/docguide/style.html
+  // with very few differences.
+  //
+  // For a list of possible configuration options, see the following page:
+  // https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md
+  // (Beware that the above looks similar but is NOT the same as the page
+  // https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md.)
+  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+  "$schema": "https://raw.githubusercontent.com/DavidAnson/markdownlint/main/schema/markdownlint-config-schema.json",
+
+  // Require ATX-style headings.
+  // https://google.github.io/styleguide/docguide/style.html#atx-style-headings
+  "headings": {
+    "style": "atx"
+  },
+
+  // Google style does not require that the first line of a file is a heading
+  // for the title; it only states that the first heading should be a level 1.
+  // https://google.github.io/styleguide/docguide/style.html#document-layout
+  "first-line-heading": false,
+
+  // The Google style does not define what to do about trailing punctuation in
+  // headings. The markdownlint default disallows exclamation points, which
+  // seems likely to be more annoying than useful – I have definitely seen
+  // people use exclamation points in headings in README files on GitHub.
+  // This setting removes exclamation point from the banned characters.
+  "no-trailing-punctuation": {
+    "punctuation": ".,;:。，；："
+  },
+
+  // No trailing spaces.
+  // https://google.github.io/styleguide/docguide/style.html#trailing-whitespace
+  "whitespace": {
+    "br_spaces": 0
+  },
+
+  // Google style is 80 characters.
+  // Google style exempts some constructs from the line-length limit:
+  // https://google.github.io/styleguide/docguide/style.html#exceptions
+  "line-length": {
+    "line_length": 80,
+    "code_block_line_length": 80,
+    "heading_line_length": 80,
+    "code_blocks": false,
+    "headings": false,
+    "tables": false
+  },
+
+  // Google Markdown style specifies 2 spaces after item numbers, 3 spaces
+  // after bullets, so that the text itself is consistently indented 4 spaces.
+  // https://google.github.io/styleguide/docguide/style.html#nested-list-spacing
+  "list-marker-space": {
+    "ol_multi": 2,
+    "ol_single": 2,
+    "ul_multi": 3,
+    "ul_single": 3
+  },
+
+  "ul-indent": {
+    "indent": 4
+  },
+
+  // Bare URLs are allowed in GitHub-flavored Markdown and in Google’s style.
+  "no-bare-urls": false,
+
+  // Basic Markdown allows raw HTML. Both GitHub & PyPI support subsets of
+  // HTML, though it's unclear what subset PyPI supports. Google's style guide
+  // recommends against using raw HTML, but does allow it. (C.f. the bottom of
+  // https://google.github.io/styleguide/docguide/style.html) Google's in-house
+  // documentation system allows many inline and block-level tags, but strips
+  // others that can pose security risks (e.g., <object> and standalone <svg>).
+  // The list below tries to capture the intersection of what GitHub allows
+  // (c.f. https://github.com/github/markup/issues/245#issuecomment-682231577),
+  // what PyPI seems to allow, what Google allows, and what seems likely to be
+  // most useful in situations where someone needs to reach for HTML.
+  "html": {
+    "allowed_elements": [
+      "a",
+      "abbr",
+      "b",
+      "blockquote",
+      "br",
+      "caption",
+      "cite",
+      "code",
+      "dd",
+      "del",
+      "details",
+      "dfn",
+      "div",
+      "dl",
+      "dt",
+      "em",
+      "figcaption",
+      "figure",
+      "h1",
+      "h2",
+      "h3",
+      "h4",
+      "h5",
+      "h6",
+      "hr",
+      "i",
+      "img",
+      "ins",
+      "kbd",
+      "li",
+      "mark",
+      "ol",
+      "p",
+      "picture",
+      "pre",
+      "q",
+      "s",
+      "samp",
+      "small",
+      "span",
+      "strong",
+      "sub",
+      "summary",
+      "sup",
+      "table",
+      "tbody",
+      "td",
+      "tfoot",
+      "th",
+      "thead",
+      "time",
+      "tr",
+      "tt",
+      "ul",
+      "var",
+      "wbr"
+    ]
+  }
+}

.yamlfmt.yaml

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# yamlfmt configuration for this project.
+# See https://github.com/google/yamlfmt for info about options.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+# Read .gitignore for paths to exclude.
+gitignore_excludes: true
+
+formatter:
+  type: basic
+  eof_newline: true
+  max_line_length: 80
+  retain_line_breaks: true
+  trim_trailing_whitespace: true

.yamllint.yaml

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Yamllint configuration to match project settings like line length.
+# See https://yamllint.readthedocs.io/ for info about configuration options.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+rules:
+  line-length:
+    max: 80
+    # When the last item on a line can't be broken (e.g., a URL as a value),
+    # it's rarely useful to flag it. The next 2 settings save developer time by
+    # not requiring comment directives to disable warnings at every occurrence.
+    allow-non-breakable-words: true
+    allow-non-breakable-inline-mappings: true