Unify multi-agent settings under agents (#33550)

## What changed

- Add `agents.enabled` as a user override for multi-agent tools while keeping an enabled `features.multi_agent_v2` authoritative.
- Rename the shared spawned-thread limit to `agents.max_concurrent_threads_per_session`, retain `agents.max_threads` as an alias, and apply the setting to both multi-agent backends.
- Add reserved subagent model, reasoning effort, and agent-type settings to the config surface and persist all resolved agent settings in config locks.
- Show the resolved `[agents]` values in TUI debug-config output.

## Testing

- Cover config loading, legacy alias normalization across merged layers, backend-selection precedence, V2 concurrency, and debug output.

GitOrigin-RevId: 2957c954a2e1aef51592768dbbccd56c4bf8f118

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

@@ -737,10 +737,10 @@ impl Session {
             TurnMultiAgentRuntime::ResolveAndStore => {
                 self.resolve_multi_agent_version_for_model(&model_info, &per_turn_config)
             }
-            TurnMultiAgentRuntime::Preview => self
-                .multi_agent_version()
-                .or(model_info.multi_agent_version)
-                .unwrap_or_else(|| per_turn_config.multi_agent_version_from_features()),
+            TurnMultiAgentRuntime::Preview => per_turn_config.multi_agent_version_for_model(
+                self.multi_agent_version()
+                    .or(model_info.multi_agent_version),
+            ),
         };
         let plugins_input = per_turn_config.plugins_config_input();
         let plugin_outcome = self