fix(FN-4811): unblock @fusion/engine typecheck so verification bootstrap can run

Symptom found while investigating 'tasks are still struggling': every
in-review task hitting pre-merge deterministic verification failed with

  [verification:bootstrap] bootstrap preamble failed (exit 2):
  [test-bootstrap] FAILED: workspace dist artifact rebuild did not complete.
  [test-bootstrap] command: pnpm --filter @fusion/engine build

Because pnpm --filter @fusion/engine build hit 17 TS errors from a prior
autonomous-agent refactor introducing a RemovalReason enum-like object
and two new audit event types. The bootstrap preamble is run by the
merger before every direct-merge verification, so a broken engine
typecheck blocked EVERY task from merging.

Fixes:

1. Duplicate RemovalReason re-export in worktree-pool.ts

   Both  and
    were present for the same identifier,
   producing TS2300 'Duplicate identifier'. RemovalReason is a const
   object with derived type (typeof-keyof pattern), so a single value
   export covers both kinds; the type-only re-export was redundant.

2. GitMutationType union missing the FN-4811 audit event types

   merger.ts and worktree-backend.ts were emitting
   'worktree:removal-refused-active-session' and
   'worktree:removal-forced-over-active-session' audit events, but the
   union in run-audit.ts didn't include them. Added both.

3. self-healing.test.ts vi.mock had wrong RemovalReason keys

   The mock only exposed 5 keys (SelfHealing*) but production code
   references HardCancel, Executor*, Merger*, PoolPrune, etc. Calls
   like removeWorktree({ reason: RemovalReason.MergerPostMerge }) were
   getting reason=undefined, producing confusing 'cannot remove
   worktree: [vitest] No RemovalReason export is defined on mock'
   error messages. Updated the mock to mirror the production const
   exactly.

4. worktree-backend.test.ts removeWorktree calls missing required reason

   The new contract makes reason: RemovalReason a required field on
   removeWorktree's input. Five existing test cases were missing it;
   added reason: RemovalReason.MergerCleanup to each.

5. integrity-warning-persisted-dedup.test.ts Settings cast

   The test's makeStore helper cast a partial settings object to
   Settings; TS rejected the narrowed type. Cast through unknown.

Verification:

  - pnpm --filter @fusion/engine build: clean
  - pnpm lint: clean
  - pnpm build (full workspace): clean
  - pnpm --filter @fusion/engine test: 5045 pass, 1 pre-existing
    aiMergeTask real-git timeout flake, 1 skipped

With this fix, the verification bootstrap can complete and the merger
can finalize tasks again.

Fusion-Task-Id: FN-4811

packages/engine/src/worktree-pool.ts

@@ -22,7 +22,7 @@ export {
   removeWorktree,
   resolveWorktreeBackend,
 } from "./worktree-backend.js";
-export type { WorktreeBackend, WorktreeBackendKind, RemovalReason } from "./worktree-backend.js";
+export type { WorktreeBackend, WorktreeBackendKind } from "./worktree-backend.js";
 export { RemovalReason } from "./worktree-backend.js";
 
 // Re-export worktrunk installer types for convenience.