feat(skill): add pr-stale-sweep skill with eval suite

[justinmclean]

Summary

Adds a new Triage-mode skill that sweeps open, non-draft pull requests for inactivity and proposes REQUEST-UPDATE or CLOSE-STALE per PR, with maintainer-court and ready-label guards. Includes 19-case eval suite and registers the skill in docs/modes.md and docs/labels-and-capabilities.md.

Generated-by: Claude (claude-sonnet-4-6)

Type of change

• Skill change (.claude/skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package (tools/*/ with pyproject.toml)
• Groovy reference impl
• Cross-cutting (RFC, AGENTS.md, sandbox, privacy-LLM)
• Documentation (docs/, README.md, CONTRIBUTING.md)
• Project template (projects/_template/)
• CI / dev loop (prek, workflows, validators)
• Other:

Test plan

• prek run --all-files passes
• For Python packages touched: uv run pytest / ruff check / mypy passes
• For Groovy bridges touched: command-line invocation tested end-to-end
• For skill changes: eval suite passes for the affected skill
  (PYTHONPATH=tools/skill-evals/src python3 -m skill_evals.runner tools/skill-evals/evals/<skill>/)
• For skill behaviour changes: a new or updated eval fixture is included in this PR
  (a regression test for the bug fixed / the behaviour added — see CONTRIBUTING.md)
• Other:

[potiuk]

Merged — thanks Justin! Rebased onto latest main and handled a few mechanical things along the way (you're preserved as commit author):

• Resolved a docs/modes.md conflict — a context collision on the Mentoring row (your branch predated the onboarding-concierge / newcomer-issue-explainer PRs). Kept main's current Mentoring row; pr-stale-sweep doesn't touch Mentoring.
• Bumped the Triage count 32 → 33. Your PR correctly bumped it 31→32 relative to its base, but another Triage skill landed on main in between, so the rebased value had to account for both.
• Regenerated docs/vendor-neutrality.md (now 69/69) — it's a hook-checked generated block and the PR predated the latest count.
• Added pr-stale-sweep to the master suites list in tools/skill-evals/README.md, where every other suite is listed.

No changes to the skill logic or evals — it's the PR-side mirror of issue-stale-sweep, cleanly structured (8 steps, a 19-case eval suite with an injection case, propose-confirm-act throughout). Nice work.