fix: prevent SQLite corruption by checkpointing WAL and closing DB on shutdown

The database was never properly closed on SIGINT/SIGTERM — WAL files were left
in a partially-written state causing "database disk image is malformed" errors.
Adds WAL checkpoint on close, synchronous=NORMAL pragma, SIGTERM handlers, and
a store.close() method that flushes all pending writes before exit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/src/store.ts

@@ -1693,6 +1693,18 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     this.recentlyWritten.clear();
   }
 
+  /**
+   * Gracefully shut down: stop watching and close the database connection.
+   * Ensures WAL is checkpointed so no pending writes are lost.
+   */
+  close(): void {
+    this.stopWatching();
+    if (this._db) {
+      this._db.close();
+      this._db = null;
+    }
+  }
+
   /**
    * Mark a file path as recently written by an in-process mutation
    * so the watcher will skip it.