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/page_route.go

@@ -0,0 +1,20 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+)
+
+// PageRoute is a simple middleware which allows you to route a static GET request
+// at the middleware stack level.
+func PageRoute(path string, handler http.Handler) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.Method == "GET" && strings.EqualFold(r.URL.Path, path) {
+				handler.ServeHTTP(w, r)
+				return
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}