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/protocol/webauthncose/ed25519.go

@@ -0,0 +1,10 @@
+package webauthncose
+
+import (
+	"crypto/ed25519"
+	"crypto/x509"
+)
+
+func marshalEd25519PublicKey(pub ed25519.PublicKey) ([]byte, error) {
+	return x509.MarshalPKIXPublicKey(pub)
+}