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/lib/tauri.ts

@@ -515,6 +515,7 @@ export async function initializeTauriListeners() {
       agent_id,
       description,
       subagent_type,
+      model,
       started_at,
       conversation_id,
       parent_tool_use_id,
@@ -526,6 +527,7 @@ export async function initializeTauriListeners() {
         agentId: agent_id,
         description,
         subagentType: subagent_type,
+        model,
         startedAt: started_at,
         status: "running",
         parentToolUseId: parent_tool_use_id,