fix: store session key in JSONL metadata to avoid lossy filename reconstruction

list_sessions() previously reconstructed the session key by replacing all
underscores in the filename with colons. This is lossy: a key like
'cli:user_name' became 'cli:user:name' after round-tripping.

Now the actual key is persisted in the metadata line during save() and read
back in list_sessions(). Legacy files without the key field fall back to
replacing only the first underscore, which handles the common channel:chat_id
pattern correctly.

Closes #899

nanobot/session/manager.py

@@ -154,6 +154,7 @@ class SessionManager:
         with open(path, "w", encoding="utf-8") as f:
             metadata_line = {
                 "_type": "metadata",
+                "key": session.key,
                 "created_at": session.created_at.isoformat(),
                 "updated_at": session.updated_at.isoformat(),
                 "metadata": session.metadata,
@@ -186,8 +187,11 @@ class SessionManager:
                     if first_line:
                         data = json.loads(first_line)
                         if data.get("_type") == "metadata":
+                            # Prefer the key stored in metadata; fall back to
+                            # filename-based heuristic for legacy files.
+                            key = data.get("key") or path.stem.replace("_", ":", 1)
                             sessions.append({
-                                "key": path.stem.replace("_", ":"),
+                                "key": key,
                                 "created_at": data.get("created_at"),
                                 "updated_at": data.get("updated_at"),
                                 "path": str(path)