refactor(sync): extract stats-sync engine behind a DB-driver interface

Move snapshot/merge/quiescence engine and ssh helpers from launcher/sync
into src/core/services/stats-sync, parameterized on a minimal SyncDb
driver. The launcher binds it to bun:sqlite via launcher/sync/bun-driver;
the sync command becomes a thin adapter over the shared sync flow.

launcher/sync/bun-driver.ts

@@ -0,0 +1,23 @@
+import { Database } from 'bun:sqlite';
+import type {
+  OpenSyncDb,
+  SyncDb,
+  SyncDbStatement,
+} from '../../src/core/services/stats-sync/driver.js';
+
+// bun:sqlite's Database.query() already caches prepared statements per SQL
+// string, which is exactly what the SyncDb contract asks for.
+export const openBunSyncDb: OpenSyncDb = (dbPath, options): SyncDb => {
+  const db = new Database(dbPath, options);
+  return {
+    query(sql: string): SyncDbStatement {
+      return db.query(sql);
+    },
+    exec(sql: string): void {
+      db.run(sql);
+    },
+    close(): void {
+      db.close();
+    },
+  };
+};