feat: add precise validation status vocabulary for reviewer reports (Closes #406)

Introduce validation_status_vocabulary with proof-path binding for baseline-
equivalent, merge-simulation-resolved, and transient-pass labels. Wire the
assessor into final_report_validator and document the taxonomy in the
review-merge workflow.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

skills/llm-project-workflow/workflows/review-merge-pr.md

@@ -568,6 +568,28 @@ If the cause is unknown, do not erase the earlier failure with plain
 `gitea_validate_review_final_report` rejects reports that omit known earlier
 validation failures when `validation_session.observed_failures` is supplied.
 
+## 21B. Validation status taxonomy (#406)
+
+When the final report summarizes how validation concluded, use one of these
+**validation status** labels (distinct from per-command pass/fail entries):
+
+* `passed` — raw PR-head validation passed on the unmodified head.
+* `failed` — raw PR-head validation failed and no allowed resolution path
+  was proven.
+* `baseline-equivalent failure accepted` — only when a clean baseline
+  worktree under `branches/` proves matching failure signatures on the target
+  branch (baseline path, target SHA, exact commands, failure lists, and
+  `failure signatures match: true`).
+* `raw-head failure resolved by merge simulation` — raw PR-head validation
+  failed, but merge simulation into the current target passed cleanly; report
+  merge simulation under `Worktree/index mutations` with full #317 proof.
+* `passed after transient failure investigation` — a later run passed after an
+  earlier failure in the same session; document the failure history (#396).
+
+Do not use `baseline-equivalent failure accepted` when only merge simulation
+resolved the failure. Do not use bare `passed` when raw PR-head validation
+failed unless one of the resolution statuses above applies.
+
 ## 22. Baseline validation rule
 
 Do not run tests in the main checkout.