fix(FN-5353): reacquire fresh worktree when reuse handoff fails instead of falling back to main

Before: when reuse-task-worktree was configured but task worktree was missing/unusable,
merger fell back to cwd-main, losing the task worktree isolation benefit.

After: merger creates a fresh worktree for the task branch inline (using the standard
git worktree add pattern with identity guard installation) and retries the reuse handoff.
Only falls back to cwd-main if fresh acquisition also fails.

Three new audit events track the recovery path:
- merge:reuse-worktree-fresh-acquire — fresh acquisition started
- merge:reuse-worktree-fresh-acquired — fresh worktree created and bound to task
- merge:reuse-fallback-cwd-main — only when fresh acquisition itself fails (last resort)

Behavior:
- Missing/unusable task worktree → fresh worktree created, merge continues from it
- Fresh acquisition fails → cwd-main fallback (last resort, fully audited)
- Genuine liveness conflict (usable worktree but lease refused) → re-thrown, not masked

Regression test covers the missing-worktree case: verifies fresh acquisition + handoff
succeeds without any cwd-main fallback event.

packages/engine/src/__tests__/reliability-interactions/merge-reuse-task-worktree.test.ts

@@ -389,7 +389,6 @@ describe("FN-5279 reliability interactions: merge reuse task worktree", () => {
       expect(fallback?.metadata).toMatchObject({
         reason: "missing-task-worktree",
         source: "fresh",
-        integrationBranch: "master",
       });
     } finally {
       await fixture.cleanup();