Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

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

vendor/github.com/go-viper/mapstructure/v2/.gitignore

@@ -0,0 +1,10 @@
+/bin/
+/build/
+/var/
+
+# Devenv
+.devenv*
+devenv.local.nix
+devenv.local.yaml
+.direnv
+.pre-commit-config.yaml