Allow issue participants to request reopening (#96947)
## Summary
- Let issue authors and people who commented before closure reopen an
issue within 14 days using `Reopen: <reason>`.
- Limit the automation to unlocked issues closed by triage-or-higher
human maintainers, with ordered timeline checks, bot filtering,
idempotency, and state revalidation.
- Align resolved-issue and contributor guidance with the new reopening
window.
## Verification
- `Prettier 3.6.2 --check .github/workflows/issue_reopen.yml
.github/comments/resolved.md contributing/repository/triaging.md`
- Mocked workflow harness covering 19 closure, eligibility, boundary,
bot, idempotency, and race scenarios
- `autoreview --mode local --stream-engine-output` (Codex and Claude
clean)
- Live GitHub API spot-check for ordered same-second `commented` and
`closed` timeline events
- Not run: live issue close/reopen smoke test (issue-triggered workflows
run only from the default branch)
