feat(messaging): implement Track 0 fault tolerance (B-14, B-15) and add 10 regression guards (G-1 to G-10)

.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job

@@ -179,7 +179,7 @@ EOF
       else
         wait "$sub_pid" 2>/dev/null
         local sub_exit=$?
-        if [ $sub_exit -eq 0 ]; then
+        if [ $sub_exit -eq 0 ] || [ $sub_exit -eq 3 ]; then
           sub_ready=1
           break
         else
@@ -337,6 +337,13 @@ EOF
         job_status="completed"
       elif [[ $sub_rc -eq 1 ]]; then
         job_status="error"
+      elif [[ $sub_rc -eq 3 ]]; then
+        job_status="broker_unavailable"
+        local disk_st
+        disk_st="$("$PY" -c "import json, os; p=os.path.join('$REGISTRY_DIR', '$JOB_ID.json'); print(json.load(open(p)).get('status','')) if os.path.exists(p) else print('')" 2>/dev/null || true)"
+        if [[ "$disk_st" == "completed" || "$disk_st" == "error" ]]; then
+          job_status="$disk_st"
+        fi
       else
         job_status="timeout"
       fi