feat: display per-agent model override in agent monitor tree

Parse the model field from Agent/Task tool input and surface it in the
agent monitor panel as a purple subtitle, so users can immediately see
which agents are running on a non-default model.

Closes #207

src-tauri/src/types.rs

@@ -292,6 +292,8 @@ pub struct AgentStartEvent {
     pub conversation_id: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub parent_tool_use_id: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub model: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]