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/github.com/spf13/cobra/MAINTAINERS

@@ -0,0 +1,13 @@
+maintainers:
+- spf13
+- johnSchnake
+- jpmcb
+- marckhouzam
+inactive:
+- anthonyfok
+- bep
+- bogem
+- broady
+- eparis
+- jharshman
+- wfernandes