fix(engine): harden merge finalize SQLite bind and recover bare merge subjects

Two compounding bugs surfaced as `feat(FN-XXXX): merge fusion/fn-XXXX`
commits landing on main:

1. The verification-fix finalize path could bind `undefined` to SQLite
   parameter 4 (`commitSha`) of `upsertTaskCommitAssociation` under the
   parallel-attempt race, failing the merge over a denormalization
   write after the commit had already landed. Centralized both
   duplicated callsites into a helper that validates each git output
   before binding.

2. Four self-healing/aiMergeTask recovery sites copied
   `classification.commit.subject` verbatim into
   `mergeDetails.mergeCommitMessage`, persisting the tier-3
   `merge ${branch}` fallback when it ended up on the landed commit.
   New `regenerateBareMergeSubject` helper detects the bare pattern
   and rebuilds a descriptive subject via the AI summarizer. Cosmetic
   only — the git commit is not amended.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/merge-finalize-sqlite-bind-and-bare-subject-recovery.md

@@ -0,0 +1,8 @@
+---
+"@fusion/engine": patch
+---
+
+Fix two bugs that compounded to produce bare `feat(FN-XXXX): merge fusion/fn-XXXX` merge commits in the dashboard:
+
+- **`Provided value cannot be bound to SQLite parameter 4` (TypeError) mid-merge**: the verification-fix finalize path called `upsertTaskCommitAssociation` with `commitSha` derived from a `git rev-parse HEAD` whose surrounding exec could reject under the parallel-attempt race, leaving `commitSha` undefined when bound to positional parameter 4. Extracted both duplicated callsites into a `recordCommitAssociationFromHead` helper that catches exec failures and validates each git output is non-empty before binding. The merge no longer fails over a denormalized lookup write when the commit itself landed cleanly.
+- **Bare-fallback subjects persisted into `mergeDetails.mergeCommitMessage`**: when `buildDeterministicMergeMessage`'s tier-3 fallback (`merge ${branch}`) made it onto a landed commit, the four `classification.commit.subject` / `landedCommit.subject` recovery sites in `self-healing.ts` and `aiMergeTask` copied that bare subject verbatim into `mergeDetails`. Added `regenerateBareMergeSubject` (in a new `merger-bare-subject.ts` module to keep self-healing's import graph narrow) which detects the bare pattern via `BARE_MERGE_SUBJECT_RE` and regenerates a descriptive subject from the landed commit's diff stat via the existing AI commit-subject summarizer. Cosmetic only — the git commit is never amended; the regenerated subject only populates the persisted `mergeDetails` and the in-process `MergeResult`. Gated by `settings.useAiMergeCommitSummary`.