Add comment-hygiene violations as a comment on the pull request

[jasonleenaylor]

Quick Summary

Comment-hygiene violations now appear in the pull request conversation, not only as
inline annotations on Files changed and in the Actions log. Follow-up to #1061.

A probe commit produced this comment, then was reverted.

• New CommentHygiene.yml scans the lines a pull request adds and posts one sticky
  comment, updated in place on each push and replaced with a clean note once the
  violations are fixed. It cannot fail a check.
• comment-hygiene.ps1 gains -ReportPath, writing the scan result as JSON;
  Build-CommentHygieneComment.ps1 renders that into the comment body.
• The scan no longer auto-runs in CI. build.ps1 and test.ps1 invoke it only for
  -CommentHygiene, making CommentHygiene.yml the only place that scans and
  annotates. That retires FW_COMMENT_HYGIENE_REPORTED and its GITHUB_ENV write, and
  moves the fetch-depth: 0 requirement to the job that diffs. Push builds, including
  the installer workflows, no longer scan.
• comment-hygiene.ps1 falls back to origin/main when origin/HEAD is unset. git rev-parse wrote to stderr, which Windows PowerShell 5.1 turns into a terminating
  error under ErrorActionPreference Stop, making the documented fallback unreachable.
• powershell-compat.ps1 walks Build/Agent instead of naming six files.
• Wording says "check" rather than "gate", and the agent requirement lives in
  AGENTS.md rather than repeated in help text that does not enforce it.

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject <= 72 chars, no trailing punctuation; if body present, blank line then <= 80-char lines).
• No whitespace warnings locally.
• Builds/tests pass locally (or I've run the CI-style build via build.ps1/test.ps1 or MSBuild).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate. No Src/** changes.

Notes for reviewers

A separate workflow, rather than steps in the build job, so the comment lands in about a
minute and still arrives when the build fails or is cancelled. Report comment hygiene
took 53s.

The comment body is passed to the sticky action as a file path, following
Build-RenderArtifactComment.ps1, so there is no GITHUB_ENV delimiter to collide with
(cf. #1073).

Verified on this pull request: Flex CI green on the shallow checkout with both Codecov
uploads passing, and the new smoke test passing under PowerShell 7 and Windows
PowerShell 5.1.

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This change is 

[github-actions]

NUnit Tests

    1 files      1 suites   11m 7s ⏱️
5 778 tests 5 697 ✅ 81 💤 0 ❌
5 787 runs  5 706 ✅ 81 💤 0 ❌

Results for commit 3abeba9.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.07%. Comparing base (d84b2b4) to head (3abeba9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #1076       +/-   ##
=========================================
+ Coverage      0   38.07%   +38.07%     
=========================================
  Files         0     1499     +1499     
  Lines         0   350154   +350154     
  Branches      0    40241    +40241     
=========================================
+ Hits          0   133315   +133315     
- Misses        0   187559   +187559     
- Partials      0    29280    +29280     

see 1499 files with indirect coverage changes

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[github-actions]

Comment hygiene (advisory)

No comment-style violations in the lines this branch adds since origin/main.