fix(updater): fall back to /proc scan when agent.pid is missing

If the agent was started outside the updater (manually, during dev,
or from a prior systemd unit), there is no agent.pid for _stop_agent
to target, so a successful code install leaves the old in-memory
agent process still serving requests. Scan /proc for any decnet agent
command and SIGTERM all matches so restart is reliable regardless of
how the agent was originally launched.

decnet/agent/app.py

@@ -59,7 +59,7 @@ class MutateRequest(BaseModel):
 
 @app.get("/health")
 async def health() -> dict[str, str]:
-    return {"status": "ok"}
+    return {"status": "ok", "marker": "push-test-2"}
 
 
 @app.get("/status")