[feat] CLI-02/C-6: argparse-style args module

[vt128]

CLI-02 / C-6 — args, a Python argparse-style argument parser

Per the maintainer's call to make argument parsing "more like Python and easy to
understand", this adds an args module whose surface deliberately mirrors
argparse:

load("args", "ArgumentParser")

p = ArgumentParser(description = "greet someone")
p.add_argument("--name", default = "World", help = "who to greet")
p.add_argument("--count", type = int, default = 1)
p.add_argument("--shout", action = "store_true")
p.add_argument("file", help = "input file")

ns = p.parse_args()                 # parses argv[1:]
print(ns.name, ns.count, ns.shout, ns.file)

$ ./starcli greet.star -- --name Kevin --count 3 --shout in.txt
Kevin 3 True in.txt

Surface

• ArgumentParser(prog, description) → add_argument(name, type, default, help, required, action="store_true") → parse_args(argv=None) → a Namespace
  struct (ns.name). Plus format_help() and args.argv.
• type accepts the Python builtins (int/float/str/bool) or the
  equivalent string.
• argv[0] is the script name (or -c), like Python sys.argv; parse_args
  defaults to argv[1:]. Handles --name val, --name=val, store_true
  flags, positionals, the -- terminator, and required checks.
• Errors read prog: error: <msg> and fail the run with a non-zero exit.

Wiring

Added to the default module set (mods.go) and classified CapPure in the
capability gate (pure parsing of the captured argv — loads in every tier).

Table-driven tests cover parsing / types / errors / the argv[1:] default / the
-- terminator / help. README documents it. Coverage 76.8% (≥ 65); Docker
golang:1.22 green.

Part of CLI-02 (STAR-19). Remaining there: stdin/pipe streaming (C-3).