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{})
+}

internal/arch/src/frontend.go

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

internal/registry/registry.go

@@ -0,0 +1,67 @@
+// Package registry provides a global registry of package-format frontends.
+//
+// Frontends self-register via init() from their own packages, so a future
+// format (e.g. Gentoo ebuild, void-src) only needs a new package that imports
+// this one — nothing else changes.
+package registry
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// Options carries per-conversion settings shared by all frontends.
+type Options struct {
+	Output string // directory to write converted output into
+	Repo   string // repository base URL (binary frontend)
+	Arch   string // target architecture (source frontend)
+}
+
+// Frontend converts one input of a specific package format to Zeta.
+type Frontend interface {
+	// Name returns the frontend's CLI subcommand name.
+	Name() string
+	// Convert converts input into Zeta format under opts.Output.
+	Convert(input string, opts Options) error
+}
+
+var (
+	mu        sync.RWMutex
+	frontends = make(map[string]Frontend)
+)
+
+// Register adds f to the global registry. It panics if a frontend with the
+// same Name is already registered.
+func Register(f Frontend) {
+	mu.Lock()
+	defer mu.Unlock()
+	name := f.Name()
+	if name == "" {
+		panic("registry: frontend with empty name")
+	}
+	if _, dup := frontends[name]; dup {
+		panic(fmt.Sprintf("registry: duplicate frontend %q", name))
+	}
+	frontends[name] = f
+}
+
+// List returns the names of all registered frontends, sorted.
+func List() []string {
+	mu.RLock()
+	defer mu.RUnlock()
+	names := make([]string, 0, len(frontends))
+	for name := range frontends {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	return names
+}
+
+// Get returns the frontend registered under name.
+func Get(name string) (Frontend, bool) {
+	mu.RLock()
+	defer mu.RUnlock()
+	f, ok := frontends[name]
+	return f, ok
+}