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/thread-manager-sample/src/main.rs

@@ -240,7 +240,11 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         project_doc_max_bytes: 32 * 1024,
         project_doc_fallback_filenames: Vec::new(),
         tool_output_token_limit: None,
+        agents_enabled: true,
         agent_max_threads: Some(6),
+        agent_support_agent_type: None,
+        agent_default_subagent_model: None,
+        agent_default_subagent_reasoning_effort: None,
         agent_job_max_runtime_seconds: None,
         agent_interrupt_message_enabled: false,
         agent_max_depth: 1,