Dispatch MCP requests concurrently per connection

handleConn processed requests serially, so a slow tool (e.g.
wait_for_pattern with a 300s timeout) monopolized the single per-agent
MCP connection and every queued call timed out behind it. Handle each
request in its own goroutine, serialize responses through a per-conn
write mutex (full response written atomically, partial writes handled),
copy the request line before handing it off (bufio reuses its buffer),
and wait on a WaitGroup before closing the conn so in-flight handlers
finish cleanly. Greeting stays sequential; notifications still get no
response.

Resolves the [MCP TIMEOUT] TODO item.

internal/mcp/mcp.go

@@ -96,10 +96,34 @@ func (s *Server) acceptLoop() {
 // identity token (SPEC §10); we resolve it to a child id and stash that
 // as the caller for every subsequent tool call.
 func (s *Server) handleConn(conn net.Conn) {
-	defer conn.Close()
+	var writeMu sync.Mutex
+	var wg sync.WaitGroup
+	defer func() {
+		wg.Wait()
+		_ = conn.Close()
+	}()
 	r := bufio.NewReader(conn)
 
 	var callerID string
+	writeResp := func(resp []byte) bool {
+		if resp == nil {
+			return true
+		}
+		resp = append(resp, '\n')
+		writeMu.Lock()
+		defer writeMu.Unlock()
+		for len(resp) > 0 {
+			n, err := conn.Write(resp)
+			if err != nil {
+				return false
+			}
+			if n == 0 {
+				return false
+			}
+			resp = resp[n:]
+		}
+		return true
+	}
 
 	greeting, err := r.ReadBytes('\n')
 	if err != nil {
@@ -115,24 +139,21 @@ func (s *Server) handleConn(conn net.Conn) {
 	} else {
 		// Treat as a real request from an unknown caller.
 		resp := s.dispatch("", greeting)
-		if resp != nil {
-			resp = append(resp, '\n')
-			if _, werr := conn.Write(resp); werr != nil {
-				return
-			}
+		if !writeResp(resp) {
+			return
 		}
 	}
 
 	for {
 		line, err := r.ReadBytes('\n')
 		if len(line) > 0 {
-			resp := s.dispatch(callerID, line)
-			if resp != nil {
-				resp = append(resp, '\n')
-				if _, werr := conn.Write(resp); werr != nil {
-					return
-				}
-			}
+			req := append([]byte(nil), line...)
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				resp := s.dispatch(callerID, req)
+				_ = writeResp(resp)
+			}()
 		}
 		if err != nil {
 			return