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/pattern_fallback.go

@@ -0,0 +1,17 @@
+//go:build !go1.23 || tinygo
+// +build !go1.23 tinygo
+
+package chi
+
+import "net/http"
+
+// supportsPattern is true if the Go version is 1.23 and above.
+//
+// If this is true, `net/http.Request` has field `Pattern`.
+const supportsPattern = false
+
+// setPattern sets the mux matched pattern in the http Request.
+//
+// setPattern is only supported in Go 1.23 and above so
+// this is just a blank function so that it compiles.
+func setPattern(rctx *Context, r *http.Request) {}