fix(reconciler): branch-fallback Phase 2 so features without prNumber reconcile

[mabry1985]

Closes #3613.

Summary

Every PR in this session's dogfood loop (#3636, #3637, #3638, #3639) sat in `[review]` until manual re-dispatch — even though the 60-second `PostMergeReconciler` poll was firing on schedule (44+ executions, 2ms each, no failures).

Root cause: three of the original stuck features (#3593, #3611, #3612) had `prNumber: null` and `prUrl: null` on disk, because the PR-creation write path (PostAgentHook recovery or normal REVIEW phase) didn't always persist the metadata back to `feature.json`. The reconciler's filter required both fields, so those features were silently skipped every tick.

Fix

`PostMergeReconcilerCheck` now runs in two phases:

Phase 1 (existing): features with `prNumber` + `prUrl` → `gh pr view` direct lookup. Transitions to `done` if merged.

Phase 2 (new): features with `branchName` but missing `prNumber` or `prUrl` → `gh pr list --head --state merged --limit 1`. When a merged PR is found, writes `prNumber`/`prUrl`/`prMergedAt` back to the feature so subsequent ticks short-circuit through Phase 1.

This is a safety-net fix — it doesn't remove the underlying need to plug the write-path gap that drops PR metadata, but it eliminates the permanent-stuck case where a feature has no way to be picked up by the reconciler.

Verification

• `npm run test:server -- tests/unit/services/post-merge-reconciler-check.test.ts` — 15/15 pass.
• `npm run test:server` — 3386 passed, 23 skipped (no regressions; +1 net from the new Phase 2 tests).
• `npm run typecheck` — server clean.
• Prettier — clean on touched files.

Test plan

• After this lands and the server restarts, any feature already stuck in `[review]` with a `branchName` (even without `prNumber`) should transition to `done` on the next poll tick (~60s).
• Dispatch a new feature through auto-mode; once its PR is merged via GitHub auto-merge, confirm the feature flips to `done` autonomously without manual re-dispatch.
• Inspect `feature.json` after the transition: it should now have `prNumber`, `prUrl`, `prMergedAt` populated (Phase 2 writes these back).

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 33da3c8a-ecf3-4259-bf74-9f0110f59818

📥 Commits

Reviewing files that changed from the base of the PR and between 356dc9d and a9159da.

📒 Files selected for processing (2)

• apps/server/src/services/maintenance/checks/post-merge-reconciler-check.ts
• apps/server/tests/unit/services/post-merge-reconciler-check.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/reconciler-branch-fallback

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Code Review — ? finding(s)

Async review running parallel to CodeRabbit. Findings are advisory; not all are merge blockers.

protoLabs Code Review Report

• Generated: 2026-05-23T07:25:21Z
• Git head: 1cfc251857f13b829290c46cee3d6f56c695b2f9
• Features mapped: 3
• Findings: 0

No findings recorded.