feat(pr-bot): coderabbit as required merge gate + auto-close (3-strike / 2-day stale)

[plind-junior]

makes CodeRabbit a hard merge gate instead of advisory, and auto-closes
PRs it keeps rejecting or that go stale. builds on #507 (which swapped
claude for coderabbit).

what changes

• coderabbit approval now gates auto-merge. a new coderabbit-gate
  workflow reads coderabbit's review verdict via the api and publishes a
  coderabbit-approved commit status. setup_repo_guards.sh adds that
  status to the branch ruleset's required checks, so a labeled non-core pr
  only auto-merges once coderabbit approves — on top of ci + trust-gate +
  codeowners. org admins keep the ruleset bypass to override when needed.
• fresh pushes can't ride a stale approval. the verdict is scoped to
  the pr head commit via each review's commit_id; a new commit with no
  coderabbit review yet reports pending, which holds the merge.
• 3 strikes and it's closed. a pr coderabbit requests changes on 3
  distinct commits without an approval is auto-closed with an explaining,
  reopenable comment.
• 2-day stale and it's closed. a daily stale-pr-reaper closes a pr
  whose author left a coderabbit change request unaddressed (no new commit)
  for 2 days. strikes catches "keeps failing"; the reaper catches
  "abandoned." the owner and bots are exempt from both.

how it's kept safe

the decision logic (coderabbit_verdict / gate_status / should_close
/ should_close_stale) lives in pr_bot.py as pure stdlib and is covered
by tests. both workflows read only review metadata and check out a trusted
ref — no untrusted head code runs; every event field is passed through
env, not interpolated into a run block.

owner follow-up

• after merge to test, re-run bash scripts/setup_repo_guards.sh test
  (and the same for main) so the ruleset actually requires the new
  coderabbit-approved check.
• the stale-pr-reaper is a schedule: workflow, which github only runs
  from the default branch — so it starts firing once this reaches
  main, not on test. (the gate itself runs on test via
  pull_request_target.)

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 440ffda5-77e2-4f83-a22e-4adf52d649b8

📥 Commits

Reviewing files that changed from the base of the PR and between d751637 and cf434a1.

📒 Files selected for processing (6)

• .coderabbit.yaml
• .github/workflows/coderabbit-gate.yml
• CHANGELOG.md
• scripts/setup_repo_guards.sh
• src/vouch/pr_bot.py
• tests/test_pr_bot.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/coderabbit-merge-gate

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