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-webauthn/webauthn/LICENSE

@@ -0,0 +1,26 @@
+Copyright (c) 2025 github.com/go-webauthn/webauthn authors.
+
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