workflow: codify PR review hygiene and reviewer context after #314 #315

Post-merge follow-on from PR #314 (git-warp v14 upgrade).

This issue captures workflow/backlog recommendations surfaced during the review cycle:

• add reviewer / CodeRabbit instructions that the published-package @git-stunts/git-warp model is canonical for this repo
• add a maintainer runbook for stale-thread resolution and false-positive handling
• add a pre-merge checklist item for unresolved review-thread cleanup

Why:
The final PR loop was slowed down more by stale bot context and unresolved outdated threads than by live code defects. We should make that cleanup path explicit so future cycles spend less time fighting review state.

Suggested acceptance criteria:

• reviewer instructions exist in a canonical, visible doc or config surface
• maintainers have a documented stale-thread / false-positive handling procedure
• pre-merge hygiene explicitly includes unresolved review-thread cleanup
• the guidance reflects the published-package dependency model and Node >= 22 runtime

Related:

• PR feat: upgrade git-warp to v14.16.2 #314
• issue dependency: audit and upgrade @git-stunts/git-warp before major Hill 1 implementation #312
• ADR-0006