shutil.move() in _load() can fail due to permissions, disk full, or
concurrent access. Without error handling, the exception propagates up
and prevents the session from loading entirely.
Wrap in try/except so migration failures are logged as warnings and the
session falls back to loading from the legacy path on next attempt.
Fixes#863
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
