fix(merger): build commit messages from actual content, not branch range

The merge commit message was built from `commitLog`/`diffStat` computed
against `merge-base(branch, main)`. Under squash-merge workflows, when an
earlier task is squash-merged onto main first, branches that forked off
the pre-squash main no longer share ancestry with it — `merge-base`
resolves to a point before the earlier task, and the message describes
work already merged via the prior squash. FN-2952's commit body claimed
11 files / 557 insertions when the actual diff was 2 files / 55 lines.

Subject was also a generic `merge <branch>` regardless of content.

- packages/engine/src/merger.ts: new `computeActualMergeCommitContext`
  helper that derives commitLog/diffStat from the actual integration
  delta (`git diff --cached <integrationTarget> --stat`), filtering
  branch commits by patch-id against the target's recent history to
  drop already-squashed siblings. Wired into both commit-finalization
  sites (`commitOrAmendMergeWithFixes` uses `preAttemptHeadSha`; the
  final amend in `runMergeAttempt` uses `HEAD~1`). Agent-context use of
  the wide range is unchanged.
- packages/engine/src/merger.ts: `buildDeterministicMergeMessage` now
  generates subject and body in parallel via `Promise.all`. Subject is
  composed as `feat(taskId): <ai summary>`, capped at 72 chars, with
  fallback to the legacy `merge <branch>` form on any AI failure.
- packages/core/src/ai-summarize.ts: new `summarizeCommitSubject` and
  `sanitizeCommitSubject` mirroring the body summarizer's structure.
  Same title-summarizer lane, 15s timeout. Sanitizer strips quotes,
  bullets, re-added conventional-commit prefixes, and trailing periods;
  hard-caps at 60 chars.
- packages/core/src/__tests__/ai-summarize.test.ts: 9 tests covering
  the sanitizer's behavior.

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

packages/core/src/index.ts

@@ -185,11 +185,16 @@ export type {
 export {
   summarizeTitle,
   summarizeCommitBody,
+  summarizeCommitSubject,
+  sanitizeCommitSubject,
   checkRateLimit,
   getRateLimitResetTime,
   validateDescription,
   SUMMARIZE_SYSTEM_PROMPT,
   COMMIT_BODY_SYSTEM_PROMPT,
+  COMMIT_SUBJECT_SYSTEM_PROMPT,
+  MAX_COMMIT_SUBJECT_LENGTH,
+  DEFAULT_COMMIT_SUBJECT_TIMEOUT_MS,
   MAX_DESCRIPTION_LENGTH,
   MIN_DESCRIPTION_LENGTH,
   MAX_TITLE_LENGTH,