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/sub_agent_dispatch_demo.rs

@@ -0,0 +1,141 @@
+//! sub_agent_dispatch_demo —— SubAgent 并行派发示例。
+//!
+//! 演示：
+//! 1. 创建父 session（"主编" agent）
+//! 2. 并行 dispatch_all 3 个子 agent（研究员 / 写手 / 审校）
+//! 3. 收集子任务结果
+//! 4. 验证树形结构：children(parent_id) 应返回 3 个子 ID
+//!
+//! 运行：`cargo run --example sub_agent_dispatch_demo`
+
+use std::sync::Arc;
+
+use agcore::agent::{Agent, AgentBuilder};
+use agcore::engine::{SessionManager, SubTaskResult};
+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 EditorAgent;
+struct ResearcherAgent;
+struct WriterAgent;
+struct ReviewerAgent;
+
+impl Agent for EditorAgent {
+    fn name(&self) -> &str {
+        "editor"
+    }
+    fn system_prompt(&self) -> Option<&str> {
+        Some("You are an editor coordinating a team.")
+    }
+}
+impl Agent for ResearcherAgent {
+    fn name(&self) -> &str {
+        "researcher"
+    }
+    fn system_prompt(&self) -> Option<&str> {
+        Some("You are a researcher. Provide 3 key findings.")
+    }
+}
+impl Agent for WriterAgent {
+    fn name(&self) -> &str {
+        "writer"
+    }
+    fn system_prompt(&self) -> Option<&str> {
+        Some("You are a writer. Draft a section.")
+    }
+}
+impl Agent for ReviewerAgent {
+    fn name(&self) -> &str {
+        "reviewer"
+    }
+    fn system_prompt(&self) -> Option<&str> {
+        Some("You are a reviewer. Check for accuracy.")
+    }
+}
+
+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(),
+    }
+}
+
+fn print_result(name: &str, r: &Result<SubTaskResult, agcore::engine::EngineError>) {
+    match r {
+        Ok(res) => println!(
+            "  ✓ {name} (child_id={}): {} tokens",
+            &res.child_id[..20.min(res.child_id.len())],
+            res.usage.total().total_tokens,
+        ),
+        Err(e) => println!("  ✗ {name}: {e}"),
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    println!("=== SubAgent Dispatch Demo ===\n");
+
+    let store: Arc<dyn agcore::memory::store::MemoryStore> = Arc::new(InMemoryStore::new());
+    let provider = Arc::new(MockProvider::new(vec![
+        assistant_text("Researcher: finding 1, 2, 3"),
+        assistant_text("Writer: section drafted"),
+        assistant_text("Reviewer: looks good"),
+    ]));
+    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"),
+    );
+
+    let editor: Arc<dyn Agent> = Arc::new(EditorAgent);
+    let researcher: Arc<dyn Agent> = Arc::new(ResearcherAgent);
+    let writer: Arc<dyn Agent> = Arc::new(WriterAgent);
+    let reviewer: Arc<dyn Agent> = Arc::new(ReviewerAgent);
+
+    let sm = Arc::new(SessionManager::new(store));
+    let parent_id = sm.create(editor, bundle.clone()).await.expect("create");
+    println!("[1] parent session created: {parent_id}");
+
+    // dispatch_all 3 个子 agent
+    println!("[2] dispatching 3 sub-agents in parallel...\n");
+    let results = sm
+        .dispatch_all(
+            &parent_id,
+            vec![
+                (researcher, "Research topic X".to_string()),
+                (writer, "Draft intro section".to_string()),
+                (reviewer, "Review draft".to_string()),
+            ],
+            agcore::engine::DispatchConfig::default(),
+        )
+        .await;
+
+    print_result("researcher", &results[0]);
+    print_result("writer", &results[1]);
+    print_result("reviewer", &results[2]);
+
+    let success_count = results.iter().filter(|r| r.is_ok()).count();
+    assert_eq!(success_count, 3, "all 3 should succeed");
+
+    // 验证树形
+    let children = sm.children(&parent_id).await.expect("children");
+    println!("\n[3] children(parent) = {} session(s)", children.len());
+    assert_eq!(children.len(), 3);
+
+    println!("\n✓ dispatch_all completed: 3/3 sub-agents succeeded");
+}