[purp]

Summary

GitHub rejects review requests for non-collaborators, including requests made individually. Move their area/topic subscriptions to WATCHERS.md and notify them through PR comment mentions, while requesting formal reviews from collaborators. Log submitted reviewer JSON and GitHub's rejection explanation so failures are actionable.

Related Issue

Closes #5539.

Verified target-repository ADMIN access; the issue has no state:accepted label.

Changes

• Move 21 memberships across 10 non-collaborators from REVIEWERS.md to WATCHERS.md, preserving every subscription and category definition.
• Mention matching watchers once per ready, classified PR, excluding the author. New labels can notify additional watchers; only trusted workflow-bot comments suppress repeats. Preview remains read-only.
• Preserve reviewer protections and runtime collaborator filtering. Log the plan before reviewer/watcher writes, exact reviewer payloads, API operation/status, and GitHub's reviewer-request error message.
• Load both rosters from the trusted default branch and protect WATCHERS.md with CODEOWNERS. Include watcher subscriptions in the labeling approval policy fingerprint.
• Update contributor, CI, and Actions troubleshooting guidance.

Release Notes

Testing

• 57 reviewer-routing tests, 28 shared acceptance tests, and 187 labeling tests pass.
• Ruff format/lint, Markdown lint, Actionlint with ShellCheck, and git diff --check pass. Routing code/tests and the changed labeling test pass ty. The labeling evidence module has two existing ty errors at line 511, reproduced on its unchanged HEAD version.
• Live collaborator audit confirms all remaining roster members are collaborators; all 21 moved subscriptions are preserved.
• Read-only preview on test(testkit): keep the JUnit report current during a run #5458 plans four eligible reviewer requests and mentions for bobbins228, gmenher, and jgarciao, with no skipped non-collaborators. It sends no requests or comments.
• A single-reviewer request for 2000krysztof on draft fix(review): notify non-collaborators through watcher mentions #5540 returned HTTP 422 with GitHub's explicit collaborator requirement. No reviewer was added.
• Independent code review found no blocking issues in watcher routing.
• Live watcher-comment delivery using the Actions token requires merging the trusted helper. Existing pull-requests: write covers PR comments; no permission increase is needed.
• E2E tests: not applicable to repository review tooling.

Checklist

• Follows Conventional Commits
• Commits have cryptographic signatures and DCO sign-offs
• Architecture docs updated: not applicable

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[purp]

This just got waaaaaay too complicated in the name of simplifying. We'll remove the watchers for now and put in a check to prevent adding someone who's not able to be requested for review. That's #5550

Note to future me: come back and figure out the more graceful way to have "watchers" who get notified even if they can't be requested for review.