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/internal/url/url.go

@@ -0,0 +1,25 @@
+package url
+
+import (
+	"errors"
+	"strings"
+)
+
+var errNoScheme = errors.New("no scheme")
+var errEmptyURL = errors.New("URL cannot be empty")
+
+// schemeFromURL returns the scheme from a URL string
+func SchemeFromURL(url string) (string, error) {
+	if url == "" {
+		return "", errEmptyURL
+	}
+
+	i := strings.Index(url, ":")
+
+	// No : or : is the first character.
+	if i < 1 {
+		return "", errNoScheme
+	}
+
+	return url[0:i], nil
+}