Persistence is lazy: getSessionFile() returns an allocated path from the
start, but the JSONL is only materialized once an assistant message (or an
explicit ensureOnDisk()) crosses the persistence gate. The shutdown banner
printed the resume hint based solely on the path, so any session that ended
before the first assistant message advertised a copy-pasteable command that
always fails with Session not found.
Gate the hint on the new SessionManager#isSessionOnDisk() (file exists in
the active storage backend) and add unit tests.
Fixes#8860
re2zero