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/pelletier/go-toml/v2/.gitignore

@@ -0,0 +1,8 @@
+test_program/test_program_bin
+fuzz/
+cmd/tomll/tomll
+cmd/tomljson/tomljson
+cmd/tomltestgen/tomltestgen
+dist
+tests/
+test-results