perf(core): opt-in in-memory SQLite for *-store tests

Adds an opt-in `inMemory` flag to `Database`/`ArchiveDatabase` (and
`{ inMemoryDb }` to TaskStore, AgentStore, RoutineStore,
AutomationStore, PluginStore) that swaps the on-disk fusion.db /
archive.db for SQLite's `:memory:` connection. Production callers
never set the flag, so behavior is unchanged.

Test files for each store now flip the flag in `beforeEach`. The
handful of tests that exercise cross-instance persistence (open store
A, close, open store B on same dir, expect data) construct disk-backed
stores explicitly inside the test body, marked with a comment at each
site.

Wall-clock impact:
- core:      69.4s → 18.5s  (3.7× faster, 3038 tests)
- dashboard: 156.6s → 30.0s (5.2× faster — improvement ripples through
                              any test that constructs a TaskStore)

The refactor eliminates the per-test SQLite open + WAL fsync + tmp
dir cleanup loop that dominated setup cost: ~50ms/test → ~5ms/test.

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

packages/core/src/routine-store.ts

@@ -67,8 +67,11 @@ export class RoutineStore extends EventEmitter<RoutineStoreEvents> {
   /** Per-routine promise chain for serializing writes. */
   private routineLocks: Map<string, Promise<void>> = new Map();
 
-  constructor(private rootDir: string) {
+  private readonly inMemoryDb: boolean;
+
+  constructor(private rootDir: string, options?: { inMemoryDb?: boolean }) {
     super();
+    this.inMemoryDb = options?.inMemoryDb === true;
   }
 
   // ── Database Access ────────────────────────────────────────────────
@@ -79,7 +82,7 @@ export class RoutineStore extends EventEmitter<RoutineStoreEvents> {
   private get db(): Database {
     if (!this._db) {
       const fusionDir = `${this.rootDir}/.fusion`;
-      this._db = new Database(fusionDir);
+      this._db = new Database(fusionDir, { inMemory: this.inMemoryDb });
       this._db.init();
     }
     return this._db;