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

.changeset/FN-4811-engine-build-unblock.md

@@ -0,0 +1,13 @@
+---
+"@runfusion/fusion": patch
+---
+
+fix(FN-4811): unblock `@fusion/engine` typecheck so verification bootstrap can run
+
+Restores `pnpm --filter @fusion/engine build` after a stack of TypeScript regressions blocked every merge. Symptoms: every task hitting pre-merge verification failed with "Verification bootstrap preamble failed — workspace dist artifact rebuild did not complete" because the bootstrap shells `pnpm --filter @fusion/engine build` and that compile was erroring on 17 type issues.
+
+Fixes:
+- Remove duplicate `RemovalReason` re-export in `worktree-pool.ts` (`export type` + `export` for the same identifier produced TS2300 "Duplicate identifier").
+- Add `worktree:removal-refused-active-session` and `worktree:removal-forced-over-active-session` to the `GitMutationType` union in `run-audit.ts` so the new FN-4811 audit events are accepted.
+- Update `self-healing.test.ts` `vi.mock("../worktree-pool.js")` to mirror the production `RemovalReason` const exactly (was missing keys, causing `reason: undefined` to flow into mock calls and confusing error messages).
+- Add `reason: RemovalReason.MergerCleanup` to existing `worktree-backend.test.ts` `removeWorktree` calls now that `reason` is a required parameter.