refactor(dashboard-agent-db): move the watch queries into their own module

Pure move: the watch, wake and batch-chain queries leave queries.ts for
watch-queries.ts, re-exported so every import path still resolves.

Author: kathiekiwi

internal-packages/dashboard-agent-db/src/internal.ts

@@ -0,0 +1,18 @@
+import { sql } from "drizzle-orm";
+import type { DashboardAgentDb } from "./client.js";
+
+// Shared by `queries.ts` and `watch-queries.ts`. Not part of the package's surface.
+
+export type DashboardAgentDbOrTx =
+  | DashboardAgentDb
+  | Parameters<Parameters<DashboardAgentDb["transaction"]>[0]>[0];
+
+/** Advisory-lock namespace (ASCII `watc`), so keys can't collide with another lock. */
+const WATCH_CHAT_LOCK_NAMESPACE = 0x77617463;
+
+/** Serializes creating a watch against deleting the chat under it. Transaction-scoped. */
+export function lockChatForWatches(tx: DashboardAgentDbOrTx, chatId: string) {
+  return tx.execute(
+    sql`select pg_advisory_xact_lock(${WATCH_CHAT_LOCK_NAMESPACE}, hashtext(${chatId}))`
+  );
+}