fix(agents): claude purge removes every path the session owns (Issue #31)

The claude purge removed only <uuid>.jsonl. It now removes everything the
session alone owns: projects/<key>/<uuid>/, file-history/<uuid>/,
session-env/<uuid>/, jobs/<uuid[:8]>/ when its state.json names the session,
a stale sessions/<pid>.json with that pid's key file (name only, never read)
and <tmp>/claude-<uid>/*/<uuid>/. An id that is empty, all dots or holds
/, .. or * builds no path. has_artifacts finds leftovers without the transcript,
and conversation_in_use also sees a live claude through sessions/<pid>.json
(a `claude -c` names no uuid on its command line); a reused pid is told
apart by ps lstart against the file's startedAt.

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

.agents/skills/lib_py/agents/base.py

@@ -84,6 +84,13 @@ def conversation_in_use(uuid: str, paths=()) -> bool:
             pass
     return False
 
+
+def _plausible_id(uuid: str) -> bool:
+    """An id safe to build a path from and to search for: not empty, not
+    made of dots alone ("." names the parent dir itself), no "/", ".." or "*"."""
+    return isinstance(uuid, str) and bool(uuid.strip(".")) and not any(s in uuid for s in ("/", "..", "*"))
+
+
 class SpawnSpec:
     def __init__(self, agent_name: str, session_name: str, command: str, env: Optional[Dict[str, str]] = None):
         self.agent_name = agent_name