test(cli): fix flaky rename watch test with deterministic waiter polling

The rename test relied on a fixed 300ms setTimeout to fire after the CLI
subprocess had registered its waiter — adequate in isolation but flaky
under full-suite load on Windows (CLI spawn + first HTTP request can
exceed 300ms). Add a tiny public MailboxStore.waiterCount(name) helper
so the test can poll until the waiter is actually registered before
triggering the rename. Also tighten the missing-name assertion from
not-zero to the contract-exact exit code 1.

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

node/src/db.ts

@@ -294,6 +294,10 @@ export class MailboxStore {
     this.waiters.delete(oldName);
   }
 
+  waiterCount(name: string): number {
+    return this.waiters.get(name)?.size ?? 0;
+  }
+
   rejectAllWaiters(): void {
     for (const bucket of this.waiters.values()) {
       for (const w of bucket) w.resolve({ kind: "aborted" });

node/tests/cli-watch.test.ts

@@ -93,13 +93,20 @@ describe("claude-mailbox watch CLI", () => {
     store.upsertMailbox("oldname");
     const child = spawn(
       process.execPath,
-      [CLI, "watch", "--block", "--name", "oldname", "--timeout", "3", "--url", baseUrl],
+      [CLI, "watch", "--block", "--name", "oldname", "--timeout", "5", "--url", baseUrl],
       { stdio: ["ignore", "pipe", "pipe"] },
     );
     let stdout = "";
     child.stdout.on("data", (d) => { stdout += d.toString(); });
 
-    setTimeout(() => store.rename("oldname", "newname"), 300);
+    // Wait for the CLI subprocess to register its waiter before renaming.
+    // A fixed delay is flaky under full-suite load on Windows.
+    const start = Date.now();
+    while (store.waiterCount("oldname") === 0) {
+      if (Date.now() - start > 4000) throw new Error("CLI never registered a waiter");
+      await new Promise((r) => setTimeout(r, 25));
+    }
+    store.rename("oldname", "newname");
 
     const code: number = await new Promise((r) => child.on("exit", (c) => r(c ?? 1)));
     expect(code).toBe(0);
@@ -116,7 +123,7 @@ describe("claude-mailbox watch CLI", () => {
 
   it("exits 1 when --name is missing", async () => {
     const r = await runCli(["watch", "--block", "--timeout", "1", "--url", baseUrl]);
-    expect(r.status).not.toBe(0);
+    expect(r.status).toBe(1);
     expect(r.stderr).toMatch(/required.*name|name.*required/i);
   });
 });