feat(ci): auto-merge pipeline, org gate, and QA fix hints
Add auto-merge workflow for org member PRs, external PR gate with
label-based approval, and actionable fix instructions for QA failures.
- auto-merge.yml: enable squash auto-merge for org member PRs
- pr-gate.yml: org-gate check blocks external PRs without label
- cmd_qa.go: add FixHint field, fixHintFor(), extractFailingTest()
- Ruleset: thread resolution, stale review dismissal, 1min merge wait
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>