feat: Invoke marker と LlmCall callback を導入し AgentTurn セマンティクスを明確化

- protocol: InvokeKind enum、Event::InvokeStart / LlmCallStart / LlmCallEnd 追加
- llm-worker: Worker.llm_call_count と on_llm_call_start/end callback、turn_count を AgentTurn 数として doc 更新
- session-store: LogEntry::Invoke { ts, trigger } 追加 (replay は marker のみで state 不変)
- pod: run/run_for_notification 開始時に Invoke marker commit、PendingRun::RunForNotification(InvokeKind) で kind を伝搬
- pod ipc: sink + server で Invoke エントリーを Event::InvokeStart として broadcast
- tui: 新 Event 3種を no-op で受理 (UI 設計はチケット範囲外)

crates/llm-worker/tests/worker_state_test.rs

@@ -352,14 +352,18 @@ async fn test_turn_count_increment() -> Result<(), WorkerError> {
     let worker = Worker::new(client);
 
     assert_eq!(worker.turn_count(), 0);
+    assert_eq!(worker.llm_call_count(), 0);
 
     // First run consumes Mutable, returns RunOutput
     let mut worker = worker.run("First").await?.worker;
     assert_eq!(worker.turn_count(), 1);
+    // Retry not yet implemented → AgentTurn:LlmCall is 1:1.
+    assert_eq!(worker.llm_call_count(), 1);
 
     // Subsequent runs on Locked take &mut self
     worker.run("Second").await?;
     assert_eq!(worker.turn_count(), 2);
+    assert_eq!(worker.llm_call_count(), 2);
 
     Ok(())
 }