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/protocol/src/protocol.rs

@@ -2005,7 +2005,7 @@ pub struct ThreadSettingsSnapshot {
     pub personality: Option<Personality>,
     pub collaboration_mode: CollaborationMode,
     #[serde(default)]
-    pub multi_agent_mode: Option<MultiAgentMode>,
+    pub multi_agent_mode: MultiAgentMode,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, Default, PartialEq, Eq, JsonSchema, TS)]