feat(cli): add Go module, subcommand dispatch, frontend registry

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

internal/arch/binary/frontend.go

@@ -0,0 +1,23 @@
+// Package binary converts Arch Linux binary packages (.pkg.tar.zst) into
+// Zeta packages.
+package binary
+
+import (
+	"errors"
+
+	"git.spectoria.dev/huntedbytheirs/zeta-reconstruct/internal/registry"
+)
+
+// frontend implements registry.Frontend for Arch binary packages.
+type frontend struct{}
+
+func (frontend) Name() string { return "arch-binary" }
+
+func (frontend) Convert(input string, opts registry.Options) error {
+	// Implemented in a later todo (.pkg.tar.zst -> tarball + package.lua).
+	return errors.New("not implemented")
+}
+
+func init() {
+	registry.Register(frontend{})
+}