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/webauthn/const.go

@@ -0,0 +1,15 @@
+package webauthn
+
+import (
+	"time"
+)
+
+const (
+	errFmtFieldNotValidDomainString = "field '%s' is not a valid domain string: %w"
+	errFmtConfigValidate            = "error occurred validating the configuration: %w"
+)
+
+const (
+	defaultTimeoutUVD = time.Millisecond * 120000
+	defaultTimeout    = time.Millisecond * 300000
+)