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/CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing
+
+## Prerequisites
+
+1. [Install Go][go-install].
+2. Download the sources and switch the working directory:
+
+    ```bash
+    go get -u -d github.com/go-chi/chi
+    cd $GOPATH/src/github.com/go-chi/chi
+    ```
+
+## Submitting a Pull Request
+
+A typical workflow is:
+
+1. [Fork the repository.][fork]
+2. [Create a topic branch.][branch]
+3. Add tests for your change.
+4. Run `go test`. If your tests pass, return to the step 3.
+5. Implement the change and ensure the steps from the previous step pass.
+6. Run `goimports -w .`, to ensure the new code conforms to Go formatting guideline.
+7. [Add, commit and push your changes.][git-help]
+8. [Submit a pull request.][pull-req]
+
+[go-install]: https://golang.org/doc/install
+[fork]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo
+[branch]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-branches 
+[git-help]: https://docs.github.com/en
+[pull-req]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests
+