Tune the unified PR review process from first-use evidence

[josecelano]

First real use of the unified PR review workflow (#2219, PR #2232) validated the core design and exposed five evidence-backed improvement candidates. Each cites its immutable reference in the audit docs/pr-reviews/pr-2232-review.md.

Tasks

1. Align the reviewer-side skill with the unified contract — review-pr/SKILL.md predates the unified workflow: advisory finding format, severity vocabulary, re-raise semantics and round-N+1 scoping for re-pushed heads (legacy friction F48), and a checklist N/A convention (legacy friction F49). Evidence: review-finding:pr-2232-f12.
2. Code-span repository-path existence check — paths in Markdown code spans are invisible to Lychee and every other gate; 21 never-existing path references and a stale agent directory map survived all checks. Needs an allowlist for intentionally historical paths. Evidence: review-finding:pr-2232-f4, review-finding:pr-2232-f5.
3. Retirement obligation inventory rule — deleting legacy skills silently dropped the only documented batch-resolve reply guard. When retiring a document, inventory its normative rules and confirm each is preserved or deliberately dropped. Evidence: review-finding:pr-2232-f6.
4. Rename-purity migration verification step — document the pattern that fixed the migration defect: diff each renamed file against its merge-base original and assert only intended lines changed. Evidence: review-finding:pr-2232-f4.
5. Tiered model routing design (design only) — a high-capability model evaluates findings and specifies bounded solutions; a lower-cost model implements them under independent verification. Recorded as a deferred automation candidate in the PR docs(pr-reviews): [#2219] unify PR review processing #2232 audit processing log.

Scope notes

• Historical audit records remain unchanged (future-only rule).
• Task 5 is a design note only; implementation is later EPIC Overhaul: Automation Tools and AI Agent Guardrails #2003 work.

A complete draft spec exists and will be formalized into docs/issues/ per the create-issue skill after PR #2232 merges.

Related to #2003. Related to #2219. Evidence source: PR #2232.