The previous implementation used `len(self._entries) > 1` to check if any
sessions had ever been created. This failed for single-platform users because
when sessions reset (via /reset, auto-reset, or gateway restart), the entry
for the same session_key is replaced in _entries, not added. So len(_entries)
stays at 1 for users who only use one platform.
Fix: Query the SQLite database's session count instead. The database preserves
historical session records (marked as ended), so session_count() correctly
returns > 1 for returning users even after resets.
This prevents the agent from reintroducing itself to returning users after
every session reset.
Fixes#351
87a16ad2e5