fix(mem0-agent): detached hooks lost their payload, so nothing was ever written

Observed live: real sessions flagged durable candidates (a preference, two
insights, a convention) but every session showed 'flush events: 0'. The manifest
backgrounded flush and assist-error with '( cmd &)', so the parent exited and
closed stdin before the child could read it -- the child then had no session_id,
fell back to 'no-session', and drained an empty buffer.

Now the CLI reads the payload itself and re-execs a detached worker with
start_new_session, so the hook still returns in ~0.27s. Verified end to end:
sent=1, session_state=created.

Also records a surface hint per event and shows it in 'sessions', since the
manifest pins MEM0_EDITOR to the same value on every client.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

integrations/mem0-agent/tests/test_cli.py

@@ -13,6 +13,8 @@ from mem0_agent.settings import SessionState, Settings
 class Args:
     def __init__(self, **kw):
         self.session_id = "sess-cli"
+        self.worker = True          # tests exercise the worker path directly
+        self.payload_file = None
         for k, v in kw.items():
             setattr(self, k, v)