feat(swarm): self-destruct agent on decommission

Decommissioning a worker from the dashboard (or swarm controller) now
asks the agent to wipe its own install before the master forgets it.
The agent stops decky containers + every decnet-* systemd unit, then
deletes /opt/decnet*, /etc/systemd/system/decnet-*, /var/lib/decnet/*,
and /usr/local/bin/decnet*. Logs under /var/log are preserved.

The reaper runs as a detached /tmp script (start_new_session=True) so
it survives the agent process being killed. Self-destruct dispatch is
best-effort — a dead worker doesn't block master-side cleanup.

decnet/swarm/client.py

@@ -185,6 +185,12 @@ class AgentClient:
         resp.raise_for_status()
         return resp.json()
 
+    async def self_destruct(self) -> dict[str, Any]:
+        """Trigger the worker to stop services and wipe its install."""
+        resp = await self._require_client().post("/self-destruct")
+        resp.raise_for_status()
+        return resp.json()
+
     # -------------------------------------------------------------- diagnostics
 
     def __repr__(self) -> str: