feat(examples): 新增 quick_start 最小可运行示例

展示 Agent / BaseTool / AgentBuilder / AgentSession 四层抽象的
最小可行集成：MockProvider + EchoTool + submit_turn("你好")，
EchoTool 真正被 LLM 调用并产出 "收到" 字样，零外部配置。

规模: 57 行（含注释），clippy 0 警告 + cargo run exit 0。

examples/quick_start.rs

@@ -0,0 +1,58 @@
+//! quick_start —— 30 行最小可运行示例，展示 Agent / BaseTool / Builder / Session 四层抽象。
+//!
+//! 运行：`cargo run --example quick_start`（离线，零配置）
+
+use std::sync::Arc;
+use agcore::agent::{Agent, AgentBuilder, AgentSession};
+use agcore::llm::hooks::HookExecutor;
+use agcore::llm::mock::MockProvider;
+use agcore::llm::provider::LlmProvider;
+use agcore::llm::types::{Usage, message::{ContentBlock, Message}, response_v2::{MessageResponse, StopReason}};
+use agcore::tools::{BaseTool, ToolContext, ToolRegistry};
+use async_trait::async_trait;
+use serde_json::{Value, json};
+
+struct Greeter;
+impl Agent for Greeter {
+    fn name(&self) -> &str { "greeter" }
+    fn system_prompt(&self) -> Option<&str> { Some("中文助手，先调用 echo 工具，再总结。") }
+}
+
+struct EchoTool;
+#[async_trait]
+impl BaseTool for EchoTool {
+    fn name(&self) -> &str { "echo" }
+    fn description(&self) -> &str { "回显文本" }
+    fn parameters(&self) -> Value {
+        json!({"type":"object","properties":{"text":{"type":"string"}},"required":["text"]})
+    }
+    async fn execute(&self, args: Value, _: &ToolContext<'_>) -> Result<Value, agcore::tools::ToolError> {
+        Ok(json!({"echoed": format!("收到: {}", args["text"].as_str().unwrap_or(""))}))
+    }
+}
+
+fn resp(content: Vec<ContentBlock>, stop: StopReason, u: (u32, u32)) -> MessageResponse {
+    MessageResponse { id: String::new(), model: "mock".into(), message: Message::Assistant { content },
+        usage: Usage::from_input_output(u.0, u.1), stop_reason: stop, extra: Default::default() }
+}
+
+#[tokio::main]
+async fn main() {
+    let mut registry = ToolRegistry::new();
+    registry.register(Arc::new(EchoTool)).unwrap();
+    let provider: Arc<dyn LlmProvider> = Arc::new(MockProvider::new(vec![
+        resp(vec![ContentBlock::ToolUse { id: "c1".into(), name: "echo".into(),
+                input: json!({"text": "你好"}) }], StopReason::ToolUse, (5, 8)),
+        resp(vec![ContentBlock::Text { text: "已通过 echo 回传您的消息。".into() }],
+             StopReason::Stop, (8, 16)),
+    ]));
+    let bundle = Arc::new(AgentBuilder::new()
+        .provider(provider).tool_registry(Arc::new(registry))
+        .hook_executor(Arc::new(HookExecutor::new())).build().unwrap());
+    let mut session = AgentSession::new(Arc::new(Greeter), "qs", bundle);
+    let resp = session.submit_turn("你好").await.unwrap();
+    let text = resp.text();
+    println!("LLM: {text}");
+    assert!(text.contains("echo") || text.contains("回传"), "响应应提及工具回传: {text}");
+    println!("\n✓ quick_start 完成");
+}