feat(mcp): Top5#3 对齐 MCP Go SDK——协议版本协商 + list_changed 差量刷新 + keepalive + Streamable HTTP

- 协议版本协商：initialize 先发最高版本(2025-11-25)并声明能力集合，遇
  UnsupportedProtocolVersionError(-32022)/服务器不支持版本 → negotiateMutuallySupportedVersion
  求出双方最高共同版本降级重试(最多2次)；无共同版本→ErrUnsupportedProtocolVersion。
  协商结果记 negotiatedVersion，HTTP 传输回填 Mcp-Protocol-Version 头。
- tools/list_changed 订阅：capabilities 声明 tools.listChanged:true；传输读无 id 帧路由到
  handleServerNotification；refreshPending 原子旗标合并突发 + 新 goroutine 调 onToolListChanged
  (避免 stdio lineRPC 重入自锁)；Registry 真差量刷新(重新 DiscoverTools→移除消失/注册新增，
  跳过同名冲突保外来工具)。
- keepalive：WithKeepAlive(interval,failures) 起 goroutine 定时 Call(ping)，连续失败≥阈值关
  session(默认3,可配)；服务器不支持 ping(-32601)停 keepalive 不误关；ctx+Close 取消不泄漏。
- Streamable HTTP 传输：newTransport 支持 TransportType=http|streamable(保留 stdio/SSE 推断，
  空值走旧逻辑不破坏)；Call POST+Accept json/event-stream 响应可单 JSON 或 SSE 流找匹配 id；
  Mcp-Session-Id 会话 + 独立 GET SSE 流收 server 通知；config 读 MCP_TRANSPORT。
- schema 缓存判定不做：客户端透传 map[string]any 无重复编译，SDK SchemaCache 是服务端
  按 Go 类型反射场景，客户端无对应。

go build/vet/test 全绿(count=2 无 flaky) + windows/darwin 交叉编译。

internal/capability/mcp/stdio.go

@@ -17,13 +17,24 @@ import (
 // stdioTransport runs an MCP server as a child process and speaks
 // newline-delimited JSON-RPC 2.0 to its stdin/stdout.
 type stdioTransport struct {
-	cfg       MCPConfig
-	log       *slog.Logger
-	line      *lineRPC
-	cmd       *exec.Cmd
-	stdin     io.WriteCloser
-	closeOnce sync.Once
-	closed    bool
+	cfg            MCPConfig
+	log            *slog.Logger
+	line           *lineRPC
+	cmd            *exec.Cmd
+	stdin          io.WriteCloser
+	closeOnce      sync.Once
+	closed         bool
+	onNotification func(method string)
+}
+
+// setOnNotification records the handler invoked when the server sends a
+// notification frame (a JSON-RPC message with no id). It is wired by the client
+// before initialize so server notifications are not lost.
+func (s *stdioTransport) setOnNotification(fn func(method string)) {
+	s.onNotification = fn
+	if s.line != nil {
+		s.line.onNotification = fn
+	}
 }
 
 // newStdioTransport builds a stdio transport for cfg.
@@ -57,7 +68,7 @@ func (s *stdioTransport) Start(ctx context.Context) error {
 	}
 	s.cmd = cmd
 	s.stdin = stdin
-	s.line = &lineRPC{in: stdin, out: bufio.NewReader(stdout), log: s.log}
+	s.line = &lineRPC{in: stdin, out: bufio.NewReader(stdout), log: s.log, onNotification: s.onNotification}
 	go s.drainStderr(stderr)
 	return nil
 }
@@ -106,11 +117,12 @@ func (s *stdioTransport) drainStderr(r io.Reader) {
 // serialized so at most one request is in flight at a time, which keeps id
 // matching simple and deterministic.
 type lineRPC struct {
-	in     io.Writer
-	out    *bufio.Reader
-	log    *slog.Logger
-	mu     sync.Mutex
-	nextID int
+	in             io.Writer
+	out            *bufio.Reader
+	log            *slog.Logger
+	mu             sync.Mutex
+	nextID         int
+	onNotification func(method string)
 }
 
 // call writes a request and reads until the matching response arrives.
@@ -151,6 +163,12 @@ func (l *lineRPC) readResponse(id, method string, result any) error {
 			continue
 		}
 		if resp.ID == nil {
+			// A server-initiated notification (no id); hand it to the client's
+			// handler if one is installed. The handler must not re-enter this
+			// transport synchronously (see the serverNotifier contract).
+			if resp.Method != "" && l.onNotification != nil {
+				l.onNotification(resp.Method)
+			}
 			continue
 		}
 		if !decodeIDString(resp, id) {