docs: add policy for abandoned community PRs

[lstein]

Summary

Adds an "Abandoned Community PRs" section to the PR Merge Policy page. It describes what to do when an outside contributor stops responding partway through review:

1. The assigned reviewer pings the author after a week with no movement, and moves on after two weeks of inactivity.
2. The reviewer decides whether the PR is worth salvaging.
3. If they adopt it, they confirm maintainer edits are allowed on the PR and get a secondary reviewer, then push fixes. The secondary reviewer approves as usual.
4. If they don't adopt it, they raise it in #request-reviews. If the group agrees to close it, the reviewer closes it with a courtesy note offering to reopen on request.

Related Issues / Discussions

None.

QA Instructions

• pnpm -C docs build succeeds, and the link validator reports all internal links valid.
• In the built page (/development/process/pr-merge-policy/), the new section renders as a five-item Starlight <Steps> list with nested bullets in steps 2, 3 and 5.

Review

No material findings.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Meaningful regression coverage added / updated where needed; obsolete tests/code removed
• Persisted-state and API changes include required migrations / compatibility validation
• Relevant performance/efficiency opportunities considered; material claims have evidence
• Material review findings resolved and relevant checks rerun
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

🤖 Generated with Claude Code

[joshistoast]

You shall pass