feat(cli): add watch --block subcommand with documented exit codes

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

node/src/cli.ts

@@ -289,6 +289,55 @@ program
     }
   });
 
+program
+  .command("watch")
+  .description(
+    "Block until one message arrives for --name, print it, and exit. Designed to be run as a Claude Code background bash task so its output surfaces via BashOutput.",
+  )
+  .requiredOption("--name <name>", "Mailbox to watch")
+  .option("--block", "Long-poll for a message (default behavior; flag accepted for clarity)")
+  .option(
+    "--timeout <seconds>",
+    "Long-poll timeout in seconds (1..300, default 25)",
+    (v) => parseInt(v, 10),
+    25,
+  )
+  .option("--url <url>", "Daemon base URL", DEFAULT_URL)
+  .action(async (opts: { name: string; block?: boolean; timeout: number; url: string }) => {
+    const url = `${opts.url}/v1/watch?name=${encodeURIComponent(opts.name)}&timeout=${opts.timeout}`;
+    let res: Response;
+    try {
+      res = await fetch(url, { headers: { "X-Mailbox": opts.name, Accept: "application/json" } });
+    } catch (err) {
+      const msg = err instanceof Error ? err.message : String(err);
+      console.error(`Could not reach daemon at ${opts.url}: ${msg}`);
+      process.exit(2);
+    }
+
+    if (res.status === 204) {
+      process.exit(3);
+    }
+
+    if (res.status === 200) {
+      const body = (await res.json()) as { from: string; body: string; sentAt: string };
+      process.stdout.write(`[Claude-Mailbox] Mail from ${body.from}:\n${body.body}\n`);
+      process.exit(0);
+    }
+
+    if (res.status === 409) {
+      const body = (await res.json().catch(() => ({}))) as { to?: string };
+      const newName = body.to ?? "<unknown>";
+      process.stdout.write(
+        `[Claude-Mailbox] Mailbox renamed to '${newName}'. Restart watcher with --name ${newName}.\n`,
+      );
+      process.exit(0);
+    }
+
+    const text = await res.text().catch(() => "");
+    console.error(`watch failed: HTTP ${res.status}${text ? ` — ${text}` : ""}`);
+    process.exit(1);
+  });
+
 program
   .command("mcp-stdio")
   .description(