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/sunset.go

@@ -0,0 +1,25 @@
+package middleware
+
+import (
+	"net/http"
+	"time"
+)
+
+// Sunset set Deprecation/Sunset header to response
+// This can be used to enable Sunset in a route or a route group
+// For more: https://www.rfc-editor.org/rfc/rfc8594.html
+func Sunset(sunsetAt time.Time, links ...string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if !sunsetAt.IsZero() {
+				w.Header().Set("Sunset", sunsetAt.Format(http.TimeFormat))
+				w.Header().Set("Deprecation", sunsetAt.Format(http.TimeFormat))
+
+				for _, link := range links {
+					w.Header().Add("Link", link)
+				}
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}