feat(FN-5633): reviewer also grades the commit message (retry-to-fix)

Add a 4th reviewer axis: the squash commit subject must concisely and
accurately summarize the actual changes. A poor/inaccurate message is an
ADVISORY-severity concern, so it drives a corrective re-merge (which rewrites
the message) up to maxReviewPasses, then lands — it never hard-blocks the merge
over a message nit. Structural correctness (task-id prefix + Fusion-Task-Id /
lineage trailers) remains deterministically guaranteed by
ensureCommitTaskMetadata regardless.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/engine/src/merger-ai.ts

@@ -280,12 +280,17 @@ export function buildReviewSystemPrompt(): string {
     "merged into the integration branch and decide whether it is safe to land.",
     "",
     "Investigate with read-only commands (git show, git diff, git log, cat, grep).",
-    "Judge on three axes:",
+    "Judge on four axes:",
     "  1. Completeness — does the squash contain ALL of the task branch's intended",
     "     changes? Flag any hunk silently dropped during conflict resolution.",
     "  2. No collateral — does it touch only files within the task's footprint?",
     "  3. Conflict soundness — were conflicts resolved coherently (both sides'",
     "     intent preserved), not by blindly discarding one side?",
+    "  4. Commit message — read `git show`'s message: the subject must concisely",
+    "     and ACCURATELY summarize the actual changes (not vague, not a mere",
+    "     restatement of the task title, not misleading). A poor/inaccurate",
+    "     message is an ADVISORY concern (it should be rewritten on retry, but",
+    "     must not block the merge).",
     "",
     "Bias toward rejection when uncertain.",
     "",