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/golang-migrate/migrate/v4/.gitignore

@@ -0,0 +1,10 @@
+.DS_Store
+cli/build
+cli/cli
+cli/migrate
+.coverage
+.godoc.pid
+vendor/
+.vscode/
+.idea
+dist/