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]>

cmd/zeta-reconstruct/main.go

@@ -0,0 +1,109 @@
+// Command zeta-reconstruct converts Arch Linux packages into Zeta package
+// format. Subcommands are provided by frontends registered in the registry.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"git.spectoria.dev/huntedbytheirs/zeta-reconstruct/internal/registry"
+
+	// Import frontend packages so their init() functions register them.
+	_ "git.spectoria.dev/huntedbytheirs/zeta-reconstruct/internal/arch/binary"
+	_ "git.spectoria.dev/huntedbytheirs/zeta-reconstruct/internal/arch/src"
+)
+
+const defaultRepo = "https://raw.githubusercontent.com/gretagen/zeta-packages/refs/heads/main"
+
+const usageText = `usage: zeta-reconstruct <command> [flags] <input>
+
+Commands:
+  arch-binary  convert an Arch binary package (.pkg.tar.zst) to Zeta
+  arch-src     convert a PKGBUILD/.SRCINFO source recipe to Zeta
+
+Flags:
+  --list       list registered frontends
+  -h, --help   show this help
+
+arch-binary flags:
+  --output <dir>  output directory (default ".")
+  --repo <url>    repository base URL (default "` + defaultRepo + `")
+
+arch-src flags:
+  --output <dir>  output directory (default ".")
+  --arch <arch>   target architecture (default "x86_64")
+`
+
+func main() {
+	os.Exit(run(os.Args[1:]))
+}
+
+func run(args []string) int {
+	if len(args) == 0 {
+		fmt.Fprintln(os.Stderr, "error: no command specified")
+		fmt.Fprintln(os.Stderr, "Run 'zeta-reconstruct --help' for usage.")
+		return 2
+	}
+
+	switch args[0] {
+	case "--list":
+		for _, name := range registry.List() {
+			fmt.Println(name)
+		}
+		return 0
+	case "--help", "-h":
+		fmt.Print(usageText)
+		return 0
+	}
+
+	frontend, ok := registry.Get(args[0])
+	if !ok {
+		fmt.Fprintf(os.Stderr, "error: unknown command %q\n", args[0])
+		fmt.Fprintln(os.Stderr, "Run 'zeta-reconstruct --help' for usage.")
+		return 2
+	}
+
+	fs := flag.NewFlagSet(frontend.Name(), flag.ContinueOnError)
+	fs.SetOutput(os.Stderr)
+	opts := registry.Options{Output: "."}
+	switch frontend.Name() {
+	case "arch-binary":
+		opts.Repo = defaultRepo
+		fs.StringVar(&opts.Repo, "repo", opts.Repo, "repository base URL")
+	case "arch-src":
+		opts.Arch = "x86_64"
+		fs.StringVar(&opts.Arch, "arch", opts.Arch, "target architecture")
+	}
+	fs.StringVar(&opts.Output, "output", opts.Output, "output directory")
+	fs.Usage = func() {
+		fmt.Fprintf(os.Stderr, "usage: zeta-reconstruct %s [flags] <input>\n", frontend.Name())
+		fmt.Fprintln(os.Stderr, "Run 'zeta-reconstruct --help' for usage.")
+	}
+
+	if err := fs.Parse(args[1:]); err != nil {
+		if err == flag.ErrHelp {
+			fs.Usage()
+			return 0
+		}
+		return 2
+	}
+
+	rest := fs.Args()
+	if len(rest) == 0 {
+		fmt.Fprintf(os.Stderr, "error: %s requires an input argument\n", frontend.Name())
+		fs.Usage()
+		return 1
+	}
+	if len(rest) > 1 {
+		fmt.Fprintf(os.Stderr, "error: unexpected argument %q\n", rest[1])
+		fs.Usage()
+		return 2
+	}
+
+	if err := frontend.Convert(rest[0], opts); err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		return 1
+	}
+	return 0
+}