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/value.go

@@ -0,0 +1,17 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+)
+
+// WithValue is a middleware that sets a given key/value in a context chain.
+func WithValue(key, val interface{}) func(next http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		fn := func(w http.ResponseWriter, r *http.Request) {
+			r = r.WithContext(context.WithValue(r.Context(), key, val))
+			next.ServeHTTP(w, r)
+		}
+		return http.HandlerFunc(fn)
+	}
+}