Parity with the gateway /reasoning handler and the new /model default:
a bare /reasoning <level> now applies to the current session only;
--global persists agent.reasoning_effort to config.yaml. --session is
still accepted as an explicit alias for the default. Display toggles
(show/hide/full/clamp) remain persistent as before — they are user
preferences, not conversation state.
Builds on YAMAGUCHI Seiji's #51158 (session-scope plumbing + /new reset,
cherry-picked as the previous commit) with the default flipped to match
the session-first policy. Fixes the CLI half of #54084.
dc0dbc9387