feat: surface last_assistant_message from SubagentStop hook payloads

Extracts last_assistant_message from SubagentStop hook events and
surfaces it in the agent monitor panel as a summary snippet below
each completed agent card.

Closes #156

src-tauri/src/types.rs

@@ -305,6 +305,8 @@ pub struct AgentEndEvent {
     pub duration_ms: Option<u64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub num_turns: Option<u32>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub last_assistant_message: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]