fix(reset,stop): warn before a reset purges, skip and prune stale registries, refuse a guessed purge

reset.sh without --yes says the conversations are deleted for good before it
asks; without a terminal the refusal says so too. A registry that cannot be
created is skipped, since nothing can be registered there, and the directories
the fence created are removed on exit when still empty. stop --purge-conversation
refuses a conversation the row does not record instead of deleting the folder's
newest one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

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

@@ -286,6 +286,11 @@ if [ -n "$TARGET_CWD" ]; then
   [ "$HERDR_ALIVE" = "1" ] && PANE_PID_NOW=$(herdr list-panes -t "$SESSION_NAME" -F '#{pane_pid}' 2>/dev/null | head -1 || true)
   eval "$(mam_session_ids stop --session "$SESSION_NAME" --agent "$AGENT" --cwd "$TARGET_CWD" \
     --herdr-alive "$HERDR_ALIVE" --pane-pid "$PANE_PID_NOW" --purge "$PURGE")" || true
+  if [ "$CAPTURE_STATUS" = "purge-unrecorded" ]; then
+    echo "ERROR: refusing to purge $SESSION_NAME; nothing was changed: $STOP_ID_NOTE." >&2
+    echo "  A stop without --purge-conversation records that id; check it is this agent's conversation, then purge." >&2
+    exit 1
+  fi
   if [ -n "$CAPTURED_UUID" ]; then echo "captured conversation id: $CAPTURED_UUID"
   else echo "WARN: no conversation id recorded before stop${STOP_ID_NOTE:+ - $STOP_ID_NOTE}"; fi
 fi