refactor: centralize cron session metadata keys

nanobot/session/keys.py

@@ -0,0 +1,12 @@
+"""Shared session key constants and helpers."""
+
+from __future__ import annotations
+
+UNIFIED_SESSION_KEY = "unified:default"
+
+
+def session_key_for_channel(channel: str, chat_id: str, *, unified_session: bool = False) -> str:
+    """Return the session key for a channel/chat pair."""
+    if unified_session:
+        return UNIFIED_SESSION_KEY
+    return f"{channel}:{chat_id}"