fix(stop): the exit-4 refusal names where the holder may be (Issue #31, Mac review)

The refusal pointed at ~/.claude/sessions/ for every agent, whatever
CLAUDE_CONFIG_DIR said, and never mentioned a non-agent process naming the
uuid on its command line. The in-use CLI now prints, on stderr while the
conversation is in use, the adapter's in_use_hints: for every adapter
`ps -axww | grep <uuid>`, and for claude its real sessions/ dir first. stop
prints them under its refusal, and says "records preserved" instead of
"no state was modified": the gate runs after the kill chain, so the session
itself may already be down. SKILL.md's exit 4 row says the same; it is not
a code block, so no inventory hash moves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/lib_py/agents/__main__.py

@@ -155,7 +155,8 @@ def main():
         # in-use <agent> <uuid> [workspace] [--wait <sec>]  (Issue #31)
         # The purge CLI's own in-use verdict, for stop_session.sh to ask
         # before it purges. Re-checks every 0.25 s for up to --wait seconds.
-        # Prints {"agent", "in_use"}. Exit 0 free; 4 still in use; 1 bad args.
+        # Prints {"agent", "in_use"}, and while in use where to look on
+        # stderr. Exit 0 free; 4 still in use; 1 bad args.
         args = sys.argv[2:]
         wait = 0.0
         if '--wait' in args:
@@ -181,6 +182,10 @@ def main():
             time.sleep(0.25)
             in_use = adapter.conversation_in_use(uuid, ctx)
         print(json.dumps({"agent": agent_name, "in_use": in_use}))
+        if in_use:
+            print("  What may hold it:", file=sys.stderr)
+            for hint in adapter.in_use_hints(uuid, ctx):
+                print(f"    {hint}", file=sys.stderr)
         sys.exit(4 if in_use else 0)
 
     else: