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

tests/data/skill_code_blocks.json

@@ -629,7 +629,7 @@
     "other"
    ],
    "passthrough": {
-    "1": "multi-agent-mux-stop/scripts/stop_session.sh:389"
+    "1": "multi-agent-mux-stop/scripts/stop_session.sh:394"
    }
   },
   "multi-agent-mux-create/scripts/create_session.sh": {