Let the worker agent run straight from a checkout

It expects worker_main.py beside it, which is true on a worker host and not in
a clone — so trying a remote worker started with copying files around. It now
looks one directory over as well.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AD8SfVhzXBG2nAfFcVh3iD

backend/app/worker/fluksio_worker.py

@@ -48,7 +48,24 @@ PROTOCOL = 1
 HEARTBEAT_S = 10.0
 #: Reconnection backs off to this and no further.
 MAX_BACKOFF_S = 30.0
-WORKER_MAIN = Path(__file__).with_name("worker_main.py")
+
+
+def _worker_main() -> Path:
+    """The user-code runner: beside this file once deployed, or in a checkout.
+
+    On a worker host the two files sit together, which is what the install
+    instructions say. Run straight from a clone and the runner is one directory
+    over, in the engine's own package — worth finding, so trying this out does
+    not start with copying files around.
+    """
+    here = Path(__file__).resolve().parent
+    for candidate in (here / "worker_main.py", here.parent / "flow" / "worker_main.py"):
+        if candidate.exists():
+            return candidate
+    return here / "worker_main.py"
+
+
+WORKER_MAIN = _worker_main()
 
 
 class Subprocess: