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"
   },

packages/engine/vitest.config.ts

@@ -48,6 +48,10 @@ export default defineConfig({
           include: ["src/**/*.test.ts"],
           exclude: [
             "src/__tests__/reliability-interactions/**/*.test.ts",
+            // Real-git heavy files run in the engine-slow project so local
+            // `pnpm test` stays snappy. CI picks them up via `test:slow`
+            // / `test:all` invoked from the root `test:full` script.
+            "src/**/*.slow.test.ts",
             "node_modules/**",
             "dist/**",
           ],
@@ -58,6 +62,9 @@ export default defineConfig({
         test: {
           name: "engine-reliability",
           include: ["src/__tests__/reliability-interactions/**/*.test.ts"],
+          // Mirror the engine-default exclusion so reliability slow tests
+          // also tier into engine-slow.
+          exclude: ["src/**/*.slow.test.ts"],
           // These tests assert event ordering across real worktrees. Parallel
           // execution under merger load caused subprocess-guard timeouts and
           // SQLite rowid interleaving (e.g. FN-5521 hit
@@ -66,6 +73,19 @@ export default defineConfig({
           poolOptions: { threads: { singleThread: true } },
         },
       },
+      {
+        extends: true,
+        test: {
+          name: "engine-slow",
+          // Files matching `*.slow.test.ts` are the long-tail real-git suites
+          // (`mkdtemp` + `git init` + multiple commits per test). They run
+          // single-threaded to avoid spawning many concurrent git processes
+          // and inflating wall time further. Excluded from the default
+          // `pnpm test` lane; run via `pnpm test:slow` / `pnpm test:all`.
+          include: ["src/**/*.slow.test.ts"],
+          poolOptions: { threads: { singleThread: true } },
+        },
+      },
     ],
     coverage: {
       enabled: false,