perf(engine): tier 4 slow real-git test files into engine-slow project

`pnpm test` was dominated by a handful of merger and reliability-
interaction files that each spawn `mkdtemp` + `git init` + multiple
commits per test. Renaming them to `*.slow.test.ts` and routing them
to a new `engine-slow` vitest project moves them out of the default
local run.

Local `pnpm test` drops from 198s to 84s (~57% faster).

- `pnpm test` — engine-default + engine-reliability lanes only
- `pnpm test:slow` — engine-slow lane (4 files, 63 tests, ~37s)
- `pnpm test:all` — everything (for CI / verify:workspace)

Files moved:
- reliability-interactions/merge-reuse-task-worktree.test.ts (was 20.6s)
- merger-overlap-guard.test.ts (was 17.1s)
- merger-staging-allowlist.test.ts (was 11.8s)
- merger-diff-volume-gate.test.ts (was 8.4s)

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

packages/engine/package.json

@@ -30,7 +30,9 @@
   "scripts": {
     "build": "tsc",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run --silent=passed-only --reporter=dot",
+    "test": "vitest run --silent=passed-only --reporter=dot --project=engine-default --project=engine-reliability",
+    "test:slow": "vitest run --silent=passed-only --reporter=dot --project=engine-slow",
+    "test:all": "vitest run --silent=passed-only --reporter=dot",
     "test:executor": "vitest run src/__tests__/executor-*.test.ts",
     "test:watch": "vitest src/__tests__/executor-*.test.ts --watch"
   },