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/agent/executor.py

@@ -115,6 +115,76 @@ def _decky_runtime_states(config: DecnetConfig) -> dict[str, dict[str, Any]]:
     return out
 
 
+_REAPER_SCRIPT = r"""#!/bin/bash
+# DECNET agent self-destruct reaper.
+# Runs detached from the agent process so it survives the agent's death.
+# Waits briefly for the HTTP response to drain, then stops services,
+# wipes install paths, and preserves logs.
+set +e
+
+sleep 3
+
+# Stop decky containers started by the local deployer (best-effort).
+if command -v docker >/dev/null 2>&1; then
+    docker ps -q --filter "label=com.docker.compose.project=decnet" | xargs -r docker stop
+    docker ps -aq --filter "label=com.docker.compose.project=decnet" | xargs -r docker rm -f
+    docker network rm decnet_lan 2>/dev/null
+fi
+
+# Stop+disable every systemd unit the installer may have dropped.
+for unit in decnet-agent decnet-engine decnet-collector decnet-forwarder decnet-prober decnet-sniffer decnet-updater; do
+    systemctl stop "$unit" 2>/dev/null
+    systemctl disable "$unit" 2>/dev/null
+done
+
+# Nuke install paths. Logs under /var/log/decnet* are intentionally
+# preserved — the operator typically wants them for forensic review.
+rm -rf /opt/decnet* /var/lib/decnet/* /usr/local/bin/decnet*
+rm -f /etc/systemd/system/decnet-*.service /etc/systemd/system/decnet-*.timer
+
+systemctl daemon-reload 2>/dev/null
+rm -f "$0"
+"""
+
+
+async def self_destruct() -> None:
+    """Tear down deckies, then spawn a detached reaper that wipes the
+    install footprint. Returns immediately so the HTTP response can drain
+    before the reaper starts deleting files out from under the agent."""
+    import os
+    import subprocess  # nosec B404
+    import tempfile
+
+    # Best-effort teardown first — the reaper also runs docker stop, but
+    # going through the deployer gives the host-macvlan/ipvlan helper a
+    # chance to clean up routes cleanly.
+    try:
+        await asyncio.to_thread(_deployer.teardown, None)
+        await asyncio.to_thread(clear_state)
+    except Exception:
+        log.exception("self_destruct: pre-reap teardown failed — reaper will force-stop containers")
+
+    # Reaper lives under /tmp so it survives rm -rf /opt/decnet*.
+    fd, path = tempfile.mkstemp(prefix="decnet-reaper-", suffix=".sh", dir="/tmp")  # nosec B108 — reaper must outlive /opt/decnet removal
+    try:
+        os.write(fd, _REAPER_SCRIPT.encode())
+    finally:
+        os.close(fd)
+    os.chmod(path, 0o700)  # nosec B103 — root-owned reaper, needs exec
+
+    # start_new_session detaches from the agent process group so the
+    # reaper isn't killed when systemctl stop decnet-agent fires.
+    subprocess.Popen(  # nosec B603
+        ["/bin/bash", path],
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+        start_new_session=True,
+        close_fds=True,
+    )
+    log.warning("self_destruct: reaper spawned path=%s — agent will die in ~3s", path)
+
+
 async def status() -> dict[str, Any]:
     state = await asyncio.to_thread(load_state)
     if state is None: