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/app-server-protocol/src/protocol/common.rs

@@ -1701,6 +1701,7 @@ mod tests {
     use codex_protocol::ThreadId;
     use codex_protocol::account::AmazonBedrockCredentialSource;
     use codex_protocol::account::PlanType;
+    use codex_protocol::config_types::MultiAgentMode;
     use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_READ_ONLY;
     use codex_protocol::parse_command::ParsedCommand;
     use codex_protocol::protocol::RealtimeConversationVersion;
@@ -2584,7 +2585,7 @@ mod tests {
                 sandbox: v2::SandboxPolicy::DangerFullAccess,
                 active_permission_profile: None,
                 reasoning_effort: None,
-                multi_agent_mode: None,
+                multi_agent_mode: MultiAgentMode::ExplicitRequestOnly,
             },
         };
 
@@ -2633,7 +2634,7 @@ mod tests {
                     },
                     "activePermissionProfile": null,
                     "reasoningEffort": null,
-                    "multiAgentMode": null
+                    "multiAgentMode": "explicitRequestOnly"
                 }
             }),
             serde_json::to_value(&response)?,