fix(codex): resolve peer-review findings R1-R6 and T1-T2

- R1: use herdr send-keys C-c for graceful stop
- R2: preserve JSONL fallback discovery when database is empty
- R3: apply workspace-scoped SQLite query before bounding results
- R4: tighten artifact validation to fail closed on missing/invalid metadata
- R5: verify actual rollout presence before reporting resumable in status
- R6: remove unconditional drift-B backend command fallback
- T1: parameterize artifact metadata regression with verified finder fixture
- T2: seed unmanaged and managed sessions in mock herdr for drift-B testing
- Add final loop review reports (100% PASS)

.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh

@@ -198,7 +198,11 @@ graceful_stop() {
   pane_pid=$(herdr list-panes -t "$SESSION_NAME" -F '#{pane_pid}' 2>/dev/null | head -1 || true)
   exitkey="$("$(_delegate_py_bin)" -m lib_py.agents exit-key "$AGENT" 2>/dev/null || echo "/exit")"
   echo "graceful: send-keys '$exitkey' to $SESSION_NAME"
-  send_keys_safe "$SESSION_NAME" "$exitkey" "stop$$" || echo "graceful: safe delivery failed (rc=$?) — falling back to kill chain"
+  if [ "$exitkey" = "Ctrl+C" ] || [ "$exitkey" = "C-c" ]; then
+    herdr send-keys -t "$SESSION_NAME" C-c 2>/dev/null || true
+  else
+    send_keys_safe "$SESSION_NAME" "$exitkey" "stop$$" || echo "graceful: safe delivery failed (rc=$?) — falling back to kill chain"
+  fi
   _wait_session_gone "$SESSION_NAME" 5 || true
   if ! herdr has-session -t "$SESSION_NAME" 2>/dev/null; then
     echo "graceful: exited cleanly"