feat(validator): add docs/modes.md consistency check (#11)

[justinmclean]

Summary

Add validate_modes_doc_consistency (SOFT, category modes-doc-consistency) to the skill-and-tool-validator. The check compares docs/modes.md against live skills/*/SKILL.md frontmatter across four dimensions:

1. Listed skill missing from disk — row exists in modes.md but skills// does not.
2. Mode mismatch — a skill's mode: frontmatter differs from the section it appears in (e.g. listed under Triage but mode: Mentoring).
3. Skill count mismatch — the integer in "Modes at a glance" Skill count column does not equal the actual row count in that mode's section.
4. Unlisted skill — a live skill has a valid mode: frontmatter value but is absent from the corresponding docs/modes.md section.

On the live tree the check surfaces two real gaps:

• reviewer-routing has mode: Triage but is absent from docs/modes.md
• good-first-issue-sweep has mode: Mentoring but is absent from docs/modes.md

Ships 19 tests in TestParseModesDocs + TestValidateModeDocConsistency; all 283 validator tests pass.

Generated-by: Claude (Opus 4.7)

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: ran over existing skills

[potiuk]

cool :)