Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/go-chi/chi/v5/middleware/maybe.go

@@ -0,0 +1,18 @@
+package middleware
+
+import "net/http"
+
+// Maybe middleware will allow you to change the flow of the middleware stack execution depending on return
+// value of maybeFn(request). This is useful for example if you'd like to skip a middleware handler if
+// a request does not satisfy the maybeFn logic.
+func Maybe(mw func(http.Handler) http.Handler, maybeFn func(r *http.Request) bool) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if maybeFn(r) {
+				mw(next).ServeHTTP(w, r)
+			} else {
+				next.ServeHTTP(w, r)
+			}
+		})
+	}
+}