docs(site): refresh IR typed-helper docs to builder-struct API

[jamesadevine]

Problem

The published site IR docs still document the old free-function task-helper API, which no longer exists in the code. After the typed-task refactor, helpers moved from src/compile/ir/tasks.rs (a single file of free functions like copy_files_step(), nuget_command_step()) to builder structs under src/compile/ir/tasks/ (one file per task, e.g. CopyFiles::new(..).into_step()).

The following site docs are stale and reference the removed API and the deleted tasks.rs file:

• site/src/content/docs/guides/extending.mdx — "The full set of typed helpers lives in src/compile/ir/tasks.rs" + free-function example.
• site/src/content/docs/reference/ir.mdx — tasks.rs bullet + the entire "Typed task helpers" table (lists copy_files_step(..), nuget_command_step(command), powershell_file_step(..), etc.).

Scope

• Update both .mdx files to describe the builder-struct API (Task::new(<required>) + typed chained setters + .into_step()), per the convention in src/compile/ir/tasks/mod.rs.
• Replace the free-function helper table with the current builder structs, and refresh the example.
• Fix references to src/compile/ir/tasks.rs -> src/compile/ir/tasks/ (directory, one file per task).
• Ensure the docs cover the full current set of builders in src/compile/ir/tasks/, including the recently-requested PublishPipelineArtifact@1, DownloadPipelineArtifact@2, and DeleteFiles@1 (the additions attempted in docs(site): add PublishPipelineArtifact@1, DownloadPipelineArtifact@2, DeleteFiles@1 to typed-helpers table #1079).

Context

• Surfaced while reviewing the open agentic-workflow PR backlog.
• docs(site): add PublishPipelineArtifact@1, DownloadPipelineArtifact@2, DeleteFiles@1 to typed-helpers table #1079 attempted to extend the stale table and was closed in favor of this full refresh.
• The docs/ir.md / docs/extending.md markdown sources may need the same treatment; check them alongside the site mdx (the big in-flight PR feat(ir): validate front-matter task steps against typed builders #1096 also touches docs/ir.md, so coordinate to avoid conflicts).