fix(stop): a purge waits for the agent process, not only the herdr session (Issue #31)

A closed herdr session is not an exited agent, and the `herdr already dead`
path ran no check at all, so stop could remove the files of a live claude
and then its row. New `python -m lib_py.agents in-use <agent> <uuid> [ws]
[--wait s]` gives the purge CLI's own verdict; stop asks it outside the
registry lock and, unless it answers free, exits 4 with nothing changed.

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

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

@@ -1,6 +1,6 @@
 # __main__.py — CLI bridge for lib_py.agents facts and resolution (N4)
 
-import os, sys, json, shlex
+import os, sys, json, shlex, time
 from lib_py.agents.registry import get_adapter, own_key, agent_of_row
 
 # `purge auto` probes adapters in this order; opencode last because its
@@ -151,6 +151,38 @@ def main():
         print(json.dumps({"agent": name, "purged": purged, "residual": residual, "in_use": False}))
         sys.exit(0)
 
+    elif cmd == 'in-use':
+        # 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.
+        args = sys.argv[2:]
+        wait = 0.0
+        if '--wait' in args:
+            i = args.index('--wait')
+            try:
+                wait = float(args[i + 1])
+            except (IndexError, ValueError):
+                print("ERROR: --wait requires seconds", file=sys.stderr)
+                sys.exit(1)
+            del args[i:i + 2]
+        agent_name = args[0] if args else ''
+        uuid = args[1] if len(args) > 1 else ''
+        workspace = args[2] if len(args) > 2 and args[2] else os.getcwd()
+        adapter = get_adapter(agent_name)
+        if not adapter or not uuid:
+            print("ERROR: in-use requires <agent> <uuid>", file=sys.stderr)
+            sys.exit(1)
+        from lib_py.agents.base import DiscoveryContext
+        ctx = DiscoveryContext(workspace, agent_name)
+        deadline = time.monotonic() + wait
+        in_use = adapter.conversation_in_use(uuid, ctx)
+        while in_use and time.monotonic() < deadline:
+            time.sleep(0.25)
+            in_use = adapter.conversation_in_use(uuid, ctx)
+        print(json.dumps({"agent": agent_name, "in_use": in_use}))
+        sys.exit(4 if in_use else 0)
+
     else:
         print(f"ERROR: Unknown CLI command {cmd!r}", file=sys.stderr)
         sys.exit(1)