Fixes the two review defects that kept PR #29923 open, plus docs:
- gateway: exclude --once from the session-store write-through. The
once-override lived only in memory before, but the write-through
persisted it, so a gateway restart before the finally-restore
rehydrated a supposedly one-turn model permanently.
- TUI: skip _sync_agent_model_with_config while a one-turn restore is
pending. The once-model is deliberately not pinned as a session
model_override, so the config sync saw a model mismatch and clobbered
the once-override back to the config model before the turn ran.
- tests: real _handle_model_command drive asserting --once never
touches set_model_override while --session still does; restore-pop
idempotency.
- docs: /model --once in configuring-models.md with an honest
prompt-cache cost note (one-shot switch breaks the cached prefix
twice; wins for short sessions and cheap-to-expensive escalation).
