[codex] Add Ultra reasoning effort (#29899)

## Why

Ultra should be one user-facing reasoning selection for work that
benefits from both maximum reasoning and proactive multi-agent
delegation. Without it, clients must coordinate maximum reasoning with
the experimental `multiAgentMode` setting, even though the inference
backend still expects its existing `max` effort value.

This change makes reasoning effort the source of truth: clients select
`ultra`, core derives proactive multi-agent behavior when the turn is
eligible for multi-agent V2, and inference requests continue to use the
backend-compatible `max` value.

## What changed

- Add `ultra` as a first-class reasoning effort and preserve
model-catalog ordering when exposing it to clients.
- Convert `ultra` to `max` at the inference request boundary, including
Responses HTTP/WebSocket requests, startup prewarm, compaction, and
memory summarization.
- Derive effective multi-agent mode per turn from effective reasoning
effort:
  - eligible multi-agent V2 + `ultra` → `proactive`
  - eligible multi-agent V2 + any other effort → `explicitRequestOnly`
- V1 or otherwise ineligible sessions → no multi-agent mode instruction
- Keep the derived effective mode in turn context history so successive
turns can emit a developer-message update only when the effective mode
changes.
- Remove selected multi-agent mode from core session configuration, turn
construction, thread settings, resume/fork restoration, and subagent
spawn plumbing. Subagents inherit reasoning effort and derive their own
effective mode.
- Retain the experimental app-server `multiAgentMode` fields for wire
compatibility while marking them deprecated. Request values are accepted
but ignored; compatibility response fields report `explicitRequestOnly`.
- Display Ultra in the TUI using the order supplied by `model/list`.

## Validation

- `just test -p codex-core ultra_reasoning_uses_max_for_requests`
- `just test -p codex-tui model_reasoning_selection_popup`

codex-rs/core/src/session/handlers.rs

@@ -124,7 +124,6 @@ async fn thread_settings_update(
         summary,
         service_tier,
         collaboration_mode,
-        multi_agent_mode,
         personality,
     } = thread_settings;
     let collaboration_mode = match collaboration_mode {
@@ -150,7 +149,6 @@ async fn thread_settings_update(
         active_permission_profile,
         windows_sandbox_level,
         collaboration_mode: Some(collaboration_mode),
-        multi_agent_mode,
         reasoning_summary: summary,
         service_tier,
         personality,
@@ -178,7 +176,6 @@ async fn thread_settings_applied_event(sess: &Session) -> EventMsg {
             reasoning_summary: snapshot.reasoning_summary,
             personality: snapshot.personality,
             collaboration_mode: snapshot.collaboration_mode,
-            multi_agent_mode: snapshot.multi_agent_mode,
         },
     })
 }