cmd/gonix: replace alecthomas/kong with stdlib flag

[aldoborrero]

The gonix CLI is small — 4 subcommands, 2 flags, 7 positional args. Well within stdlib flag's reach.

Approach

• main.go dispatches on os.Args[1] to nar.Main() / drv.Main()
• Each subpackage's Main() builds a flag.FlagSet per subcommand and reads positionals from fs.Args()
• The custom drv-store-uri kong mapper becomes a plain derivationStore.NewFromURI call after parsing

Per-subcommand files (cat.go, dump.go, ls.go, show.go) keep their Cmd structs and Run() methods unchanged — only the kong struct tags are stripped. Help text and arg-count validation become explicit.

Behavior preserved

$ gonix nar dump-path cmd/gonix/nar > /tmp/test.nar
$ gonix nar ls -R /tmp/test.nar
-r-xr-xr-x /
-r--r--r-- /cat.go (907 bytes)
...
$ gonix nar cat /tmp/test.nar /cmd.go | head -1
package nar
$ gonix drv show --format aterm /nix/store/...drv
Derive([("out",...

Error paths (unknown command, missing positionals, --help) all produce sensible output.

Dependency reduction

Drops alecthomas/kong and transitively alecthomas/repr + pkg/errors from go.sum. First of a small series trimming the dependency surface.