feat(engine): 实现 Agent 角色热切换与子代理调度

新增 SessionManager::switch_agent / dispatch / dispatch_all / dispatch_stream
四个核心方法，补齐多 Agent 基础系统原语。

交付物:
- switch_agent 运行时替换 Arc<dyn Agent>，保留上下文并更新 SessionMeta
- dispatch 单任务派发：create_child → inherit_memory → submit_turn
- dispatch_all 并行派发：Semaphore 并发控制 + 部分成功语义
- dispatch_stream 流式派发：unbounded_channel + spawn task 消息重建 + finalize_turn
- DispatchConfig / SubTaskResult / SubTaskStreamEvent 公开类型
- 4 个端到端示例（agent_switch_demo / sub_agent_dispatch_demo /
  bridge_keys_demo / dispatch_stream_demo）

辅助变更:
- EngineError 新增 DispatchFailed 变体
- CostTracker 实现 From<Usage> 转换
- save_session_meta / load_session_meta 改 pub(crate) 供 switch.rs 使用

测试: +17 个内联测试（4 switch + 5 dispatch + 4 dispatch_all +
4 dispatch_stream），全量 391 passed / 0 failed，
clippy 0 警告，doc 0 warning。

examples/agent_switch_demo.rs

@@ -0,0 +1,115 @@
+//! agent_switch_demo —— Agent 角色热切换示例。
+//!
+//! 演示：
+//! 1. 创建 session（绑定 Analyst agent）
+//! 2. 提交一轮对话（角色 A 输出"分析数据"）
+//! 3. switch_agent 切换为 Reporter agent
+//! 4. 提交第二轮对话（角色 B 基于已有上下文输出"报告"）
+//! 5. 验证：turn_index 连续、session_memory 保留、slot 历史保留
+//!
+//! 运行：`cargo run --example agent_switch_demo`
+
+use std::sync::Arc;
+
+use agcore::agent::{Agent, AgentBuilder};
+use agcore::engine::SessionManager;
+use agcore::llm::hooks::HookExecutor;
+use agcore::llm::mock::MockProvider;
+use agcore::llm::types::Usage;
+use agcore::llm::types::message::{ContentBlock, Message};
+use agcore::llm::types::response_v2::{MessageResponse, StopReason};
+use agcore::memory::store::InMemoryStore;
+use agcore::tools::ToolRegistry;
+
+struct AnalystAgent;
+struct ReporterAgent;
+
+impl Agent for AnalystAgent {
+    fn name(&self) -> &str {
+        "analyst"
+    }
+    fn system_prompt(&self) -> Option<&str> {
+        Some("You are a data analyst. Analyze the input concisely.")
+    }
+}
+
+impl Agent for ReporterAgent {
+    fn name(&self) -> &str {
+        "reporter"
+    }
+    fn system_prompt(&self) -> Option<&str> {
+        Some("You are a report writer. Write concise reports based on context.")
+    }
+}
+
+fn assistant_text(text: &str) -> MessageResponse {
+    MessageResponse {
+        id: String::new(),
+        model: String::new(),
+        message: Message::Assistant {
+            content: vec![ContentBlock::Text { text: text.into() }],
+        },
+        usage: Usage::from_input_output(8, 4),
+        stop_reason: StopReason::Stop,
+        extra: Default::default(),
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    println!("=== Agent Switch Demo ===\n");
+
+    // 1. 准备组件
+    let store: Arc<dyn agcore::memory::store::MemoryStore> = Arc::new(InMemoryStore::new());
+    let provider = Arc::new(MockProvider::new(vec![
+        assistant_text("Analyst: data analyzed (Q3 sales up 15%)"),
+        assistant_text("Reporter: report drafted (3 paragraphs)"),
+    ]));
+    let bundle = Arc::new(
+        AgentBuilder::new()
+            .provider(provider)
+            .tool_registry(Arc::new(ToolRegistry::new()))
+            .hook_executor(Arc::new(HookExecutor::new()))
+            .session_memory_backend(store.clone())
+            .build()
+            .expect("RuntimeBundle 装配失败"),
+    );
+
+    let analyst: Arc<dyn Agent> = Arc::new(AnalystAgent);
+    let reporter: Arc<dyn Agent> = Arc::new(ReporterAgent);
+
+    let sm = Arc::new(SessionManager::new(store));
+    let session_id = sm.create(analyst, bundle.clone()).await.expect("create");
+    println!("[1] session created: {session_id}");
+
+    // 2. Analyst 跑一轮
+    let resp1 = sm
+        .submit_turn(&session_id, "Analyze Q3 sales data")
+        .await
+        .expect("submit_turn 1");
+    println!("[2] analyst turn 1: {:?}", resp1.text());
+
+    // 3. 切换到 Reporter
+    sm.switch_agent(&session_id, reporter)
+        .await
+        .expect("switch_agent");
+    println!("[3] agent switched to 'reporter'");
+
+    // 4. Reporter 跑一轮（基于已有上下文）
+    let resp2 = sm
+        .submit_turn(&session_id, "Write a report based on the analysis")
+        .await
+        .expect("submit_turn 2");
+    println!("[4] reporter turn 2: {:?}", resp2.text());
+
+    // 5. 验证 turn_index 连续
+    let (turn_index, agent_name_owned) = {
+        let session = sm.get(&session_id).await.unwrap();
+        let guard = session.lock().await;
+        (guard.turn_index(), guard.agent.name().to_string())
+    };
+    println!("\n[verify] turn_index = {turn_index}, agent = {agent_name_owned}");
+    assert_eq!(turn_index, 2, "turn_index should be 2 after 2 turns");
+    assert_eq!(agent_name_owned, "reporter", "current agent should be reporter");
+    println!("✓ context preserved across agent switch");
+}