feat(cli): pre-install port check in install-autostart

Before registering the Scheduled Task / Run-key / launchd / systemd unit,
probe /health on the resolved port. If a non-claude-mailbox service
answers, refuse with a helpful hint (`--port <n>` or mailbox.json) so
users don't end up with autostart firing against an occupied port.
Pass --skip-port-check to bypass.

The doctor already had this logic in Step 2; now standalone
install-autostart invocations are protected too.

node/src/cli.ts

@@ -345,11 +345,49 @@ program
   .option("--port <port>", "Port to listen on", (v) => parseInt(v, 10))
   .option("--bind <address>", "Bind address")
   .option("--db-path <path>", "SQLite database path")
-  .action(async (opts: { service?: boolean; port?: number; bind?: string; dbPath?: string }) => {
+  .option(
+    "--skip-port-check",
+    "Skip the pre-install probe for a foreign occupant on the daemon's port",
+  )
+  .action(
+    async (opts: {
+      service?: boolean;
+      port?: number;
+      bind?: string;
+      dbPath?: string;
+      skipPortCheck?: boolean;
+    }) => {
+      if (!opts.skipPortCheck) {
+        const cfg = resolveConfig({ port: opts.port, bind: opts.bind, dbPath: opts.dbPath });
+        const probeUrl = `http://${cfg.bind}:${cfg.port}/health`;
+        try {
+          const res = await fetch(probeUrl, { headers: { Accept: "application/json" } });
+          const text = await res.text();
+          let parsed: { status?: string; version?: string } | null = null;
+          try {
+            parsed = text.length ? (JSON.parse(text) as { status?: string; version?: string }) : null;
+          } catch {
+            parsed = null;
+          }
+          if (res.ok && parsed?.status === "ok" && parsed.version) {
+            console.log(
+              `Port ${cfg.port} already serves a claude-mailbox daemon (version ${parsed.version}). Autostart will manage that one.`,
+            );
+          } else {
+            console.error(
+              `Port ${cfg.port} is held by a non-claude-mailbox service (status ${res.status}). Pick a free port via \`--port <n>\` or write {"port": <n>} to ~/.claude-mailbox/mailbox.json. Use --skip-port-check to bypass.`,
+            );
+            process.exit(4);
+          }
+        } catch {
+          // Connection refused or similar — port is free, proceed.
+        }
+      }
       const mgr = await autostartManager(opts.service ? "service" : "default");
       await mgr.install({ port: opts.port, bind: opts.bind, dbPath: opts.dbPath });
       console.log("Autostart installed.");
-  });
+    },
+  );
 
 program
   .command("uninstall-autostart")