Add AGENTS.md

AGENTS.md

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+# AGENTS.md
+
+## Repository Purpose
+This is a sandbox/test repository used to experiment with Sourcegraph Batch Changes.
+It contains no production code. Do not treat it as a functional project.
+
+## Language
+All content is Markdown. There is no compiled or interpreted source code.
+
+## Directory Structure
+- `.github/workflows/blank.yml` — placeholder GitHub Actions CI (non-functional)
+- `*.md` — various Markdown files used as targets for batch change experiments
+
+## Build, Test, and Lint
+There are no build, test, or lint commands. The CI workflow is a stub and performs no
+meaningful checks. Do not attempt to run tests or linters — none exist.
+
+## Making Changes
+- All changes should be to `.md` files.
+- There is no review process defined beyond the stub `CONTRIBUTING.md`.
+- PRs targeting the `main` branch trigger the placeholder CI workflow, which
+  always passes regardless of content.
+
+## Key Constraints for Agents
+- Do not add source code files; this repo is Markdown-only by design.
+- Do not rely on CI green status as a signal of correctness — the workflow is a no-op.
+- The `CHANGELOG.md`, `CONTRIBUTING.md`, and `TESTING.md` files are stubs; do not
+  treat them as authoritative documentation.