fix: fix: check_active guard should log to stderr when skipping — silent agent dropout on missing state file (#663)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/guard.sh

@@ -15,6 +15,7 @@ check_active() {
   local agent_name="$1"
   local state_file="${FACTORY_ROOT}/state/.${agent_name}-active"
   if [ ! -f "$state_file" ]; then
+    echo "[check_active] SKIP: state file state/.${agent_name}-active not found — agent disabled" >&2
     log "${agent_name} not active — skipping"
     exit 0
   fi