Show spawned agent model and effort in TUI (#14273)

- include the requested sub-agent model and reasoning effort in the
spawn begin event\n- render that metadata next to the spawned agent name
and role in the TUI transcript

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/protocol.rs

@@ -3132,6 +3132,8 @@ pub struct CollabAgentSpawnBeginEvent {
     /// Initial prompt sent to the agent. Can be empty to prevent CoT leaking at the
     /// beginning.
     pub prompt: String,
+    pub model: String,
+    pub reasoning_effort: ReasoningEffortConfig,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]