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/agent-store.ts

@@ -82,6 +82,12 @@ export interface AgentStoreOptions {
   rootDir?: string;
   /** Optional TaskStore for checkout/release operations */
   taskStore?: TaskStore;
+  /**
+   * Test-only: open the underlying SQLite DB as `:memory:` instead of a
+   * disk-backed file. Skips per-test fsync and WAL setup; mirrors the
+   * pattern in TaskStore. Production callers must leave this unset.
+   */
+  inMemoryDb?: boolean;
 }
 
 /** Agent data as stored in SQLite JSON columns */
@@ -174,6 +180,7 @@ export class AgentStore extends EventEmitter {
   private locks: Map<string, AgentLock> = new Map();
   private _db: Database | null = null;
   private taskStore?: TaskStore;
+  private readonly inMemoryDb: boolean;
 
   constructor(options: AgentStoreOptions = {}) {
     super();
@@ -187,11 +194,12 @@ export class AgentStore extends EventEmitter {
     this.rootDir = options.rootDir ?? resolve(".fusion");
     this.agentsDir = join(this.rootDir, "agents");
     this.taskStore = options.taskStore;
+    this.inMemoryDb = options.inMemoryDb === true;
   }
 
   private get db(): Database {
     if (!this._db) {
-      this._db = new Database(this.rootDir);
+      this._db = new Database(this.rootDir, { inMemory: this.inMemoryDb });
       this._db.init();
     }
     return this._db;