Make async git checkout return immediately

Once the job is dispatched the agent cloned synchronously even with
async=1, so a slow clone blew past the controller's 5s dispatch timeout
and the job was marked failed while the worker kept building. Run the
clone in the background thread and return 202 + job id right away.

agent/build_agent.py

@@ -228,15 +228,18 @@ def checkout_project():
     build_root = os.path.join(TMP_DIR, BUILD_DIR)
     repo_dir = os.path.join(build_root, job_id)
     start_time = datetime.datetime.now()
+    if async_mode:
+        os.makedirs(build_root, exist_ok=True)
+        _set_step(job_id, "cloning_repo", repo_url=repo_url)
+        system_status['status'] = "cloning_repo"
+        _run_checkout_build_background(job_id, repo_url, repo_dir, start_time)
+        return jsonify({"id": job_id, "async": True, "message": "Build started"}), 202
     try:
         os.makedirs(build_root, exist_ok=True)
         _set_step(job_id, "cloning_repo", repo_url=repo_url)
         system_status['status'] = "cloning_repo"
         subprocess.check_call(['git', 'clone', repo_url, repo_dir])
         system_status['status'] = "ready"
-        if async_mode:
-            _run_build_background(job_id, repo_dir, start_time)
-            return jsonify({"id": job_id, "async": True, "message": "Build started"}), 202
         return install_and_build(repo_dir, job_id, start_time)
     except Exception as e:
         print(f"Error processing checkout: {e}")
@@ -251,6 +254,25 @@ def checkout_project():
         if not async_mode:
             build_lock.release()
 
+
+def _run_checkout_build_background(job_id, repo_url, repo_dir, start_time):
+    """Run git clone + install_and_build in a background thread so async
+    checkouts return immediately. Assumes build_lock is held; the thread
+    releases it when the work finishes."""
+    def runner():
+        try:
+            subprocess.check_call(['git', 'clone', repo_url, repo_dir])
+            install_and_build(repo_dir, job_id, start_time)
+        except Exception as e:
+            print(f"Background checkout/build failed for {job_id}: {e}")
+            system_status['status'] = "ready"
+            system_status['running_job'] = None
+            _clear_progress(job_id)
+            shutil.rmtree(repo_dir, ignore_errors=True)
+        finally:
+            build_lock.release()
+    threading.Thread(target=runner, name=f"checkout-{job_id}", daemon=True).start()
+
 def _run_build_background(job_id, project_path, start_time):
     """Run install_and_build in a background thread. Assumes build_lock is held;
     the thread releases it when the build finishes."""