chore: import upstream snapshot with attribution

pkg/http/transport/user_agent.go

@@ -0,0 +1,18 @@
+package transport
+
+import (
+	"net/http"
+
+	"github.com/github/github-mcp-server/pkg/http/headers"
+)
+
+type UserAgentTransport struct {
+	Transport http.RoundTripper
+	Agent     string
+}
+
+func (t *UserAgentTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req = req.Clone(req.Context())
+	req.Header.Set(headers.UserAgentHeader, t.Agent)
+	return t.Transport.RoundTrip(req)
+}