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/log.go

@@ -0,0 +1,12 @@
+package migrate
+
+// Logger is an interface so you can pass in your own
+// logging implementation.
+type Logger interface {
+
+	// Printf is like fmt.Printf
+	Printf(format string, v ...interface{})
+
+	// Verbose should return true when verbose logging output is wanted
+	Verbose() bool
+}