Simplify multi-agent mode controls (#29324)

## Why

Multi-agent delegation policy was split across `multiAgentMode`,
`features.multi_agent_mode`, and `usage_hint_enabled`. These controls
could disagree: a requested mode could be downgraded by the feature
flag, and disabling usage hints also disabled mode instructions.

Some clients also need multi-agent tools without adding
delegation-policy text to model context. The previous two-mode API could
not express that directly.

## What changed

`multiAgentMode` is now the only live delegation-policy control:

| Mode | Behavior |
| --- | --- |
| `none` | Keep multi-agent tools available without adding mode
instructions. |
| `explicitRequestOnly` | Only delegate after an explicit user request.
|
| `proactive` | Delegate when parallel work materially improves speed or
quality. |

- new threads default to `explicitRequestOnly`; omitting the mode on
later turns keeps the current value
- thread start, resume, fork, and settings responses always report the
concrete current mode instead of `null`
- mode selection remains sticky across turns and resume
- usage-hint text no longer controls whether mode instructions apply
- `features.multi_agent_mode` and `usage_hint_enabled` remain accepted
as ignored compatibility settings so existing configs continue to load
- app-server documentation and generated schemas describe the three-mode
API

## Tests

- `just test -p codex-core multi_agent_mode`
- `just test -p codex-core multi_agent_v2_config_from_feature_table`
- `just test -p codex-core spawn_agent_description`
- `just test -p codex-features`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server multi_agent_mode`

codex-rs/features/src/tests.rs

@@ -604,45 +604,6 @@ non_code_mode_only = true
     );
 }
 
-#[test]
-fn multi_agent_v2_feature_config_usage_hint_enabled_does_not_enable_feature() {
-    let features_toml: FeaturesToml = toml::from_str(
-        r#"
-[multi_agent_v2]
-usage_hint_enabled = false
-"#,
-    )
-    .expect("features table should deserialize");
-    let features = Features::from_sources(
-        FeatureConfigSource {
-            features: Some(&features_toml),
-            ..Default::default()
-        },
-        FeatureConfigSource::default(),
-        FeatureOverrides::default(),
-    );
-
-    assert_eq!(features.enabled(Feature::MultiAgentV2), false);
-    assert_eq!(features_toml.entries(), BTreeMap::new());
-    assert_eq!(
-        features_toml.multi_agent_v2,
-        Some(crate::FeatureToml::Config(crate::MultiAgentV2ConfigToml {
-            enabled: None,
-            max_concurrent_threads_per_session: None,
-            min_wait_timeout_ms: None,
-            max_wait_timeout_ms: None,
-            default_wait_timeout_ms: None,
-            usage_hint_enabled: Some(false),
-            usage_hint_text: None,
-            root_agent_usage_hint_text: None,
-            subagent_usage_hint_text: None,
-            tool_namespace: None,
-            hide_spawn_agent_metadata: None,
-            non_code_mode_only: None,
-        }))
-    );
-}
-
 #[test]
 fn materialize_resolved_enabled_writes_all_features_and_preserves_custom_config() {
     let mut features = Features::with_defaults();