Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/libc/CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing to this repository
+
+Thank you for your interest in contributing! To help keep the project stable across its many targets, please follow these guidelines when submitting a pull request or merge request.
+
+### Verification
+
+Before submitting your changes, please ensure that they do not break the build for different architectures or build tags. 
+
+Run the following script in your local environment:
+
+```bash
+    $ ./build_all_targets.sh
+```
+
+Please verify that all targets you can test pass before opening your request.
+
+### Authors and Contributors
+
+If you would like yourself and/or your company to be officially recognized in the project:
+
+ * Optionally, please include a change to the AUTHORS and/or CONTRIBUTORS files within your merge request.
+
+### The Process
+
+ * Fork the repository (or host a public branch if you do not have a gitlab.com account).
+ * Implement your changes, keeping them as focused as possible.
+ * Submit your request with a clear description of the problem solved, the dependency improved, etc.
+
+----
+
+We appreciate your help in making the Go ecosystem more robust!