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
73530d51ac