feat(agent-queue): refuse a second run when a daemon is already active

cmd_run now checks daemon.pid liveness up front: if a run loop is alive it exits
with an error (protecting the single-launcher invariant locking depends on); a
stale daemon.pid (dead pid) is cleared and the run proceeds.

agent-queue/agent-queue.sh

@@ -340,6 +340,14 @@ cmd_run() {
       *) die "run: unknown arg '$1'";;
     esac
   done
+  # Refuse to start a second run loop against the same queue — two daemons would
+  # break the single-launcher invariant that per-cwd locking relies on.
+  local dpid=""
+  [[ -f "$STATE/daemon.pid" ]] && dpid=$(cat "$STATE/daemon.pid" 2>/dev/null)
+  if [[ -n "$dpid" ]] && kill -0 "$dpid" 2>/dev/null; then
+    die "a run loop is already active (pid $dpid). Use 'stop' first, or a different AGENT_QUEUE_ROOT."
+  fi
+  [[ -n "$dpid" ]] && log "clearing stale daemon.pid ($dpid)"
   echo "$$" > "$STATE/daemon.pid"
   trap 'rm -f "$STATE/daemon.pid"; log "run loop stopped"; exit 0' INT TERM
   log "run loop started (max=$MAX_CONCURRENCY, default engine=$DEFAULT_ENGINE). Ctrl-C to stop."