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/path_rewrite.go

@@ -0,0 +1,16 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+)
+
+// PathRewrite is a simple middleware which allows you to rewrite the request URL path.
+func PathRewrite(old, new string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			r.URL.Path = strings.Replace(r.URL.Path, old, new, 1)
+			next.ServeHTTP(w, r)
+		})
+	}
+}