* fix(fast): default /fast to session scope on CLI and gateway
Completes the session-first policy from #67946 for the /fast toggle
(the remaining half of #54084). A bare /fast fast|normal now applies to
the current session only; --global persists agent.service_tier to
config.yaml.
Gateway: new _session_service_tier_overrides dict (registered in
_CONVERSATION_SCOPED_STATE so /new clears it) resolved at both agent
turn sites via _resolve_session_service_tier(); the /fast handler and
its choice picker apply session overrides and evict the cached agent.
The TUI config.set fast path was already session-scoped.
CLI: /fast parses --global (parity with /reasoning); bare toggles
mutate self.service_tier only.
* fix(sessions): /new resets model, reasoning, and fast to config defaults
/new and /reset are full conversation boundaries: session-scoped
runtime overrides do not carry into the next session (#48055, #23131).
CLI new_session(): clears the one-turn model restore, re-derives
service_tier from config, and — when the session's model differs from
the config default — switches back via the shared switch_model()
pipeline (live agent swap included; best-effort so an unreachable
default never blocks /new).
TUI _reset_session_agent(): stops forwarding model_override /
create_reasoning_override / create_service_tier_override into the
rebuilt agent and pops the pins so later rebuilds can't resurrect
them. The gateway already cleared its per-session overrides via
_clear_conversation_scope on /new.
Cross-session contamination stays impossible: nothing here touches
process-global env or other sessions' pins.
47fb20c0bd