fix(setup): support -h/--help instead of running installer (#1133)

[saen-ai]

Fixes #1133.

Before

./setup --help silently ran the full installer because the arg parser's catch-all (*) shift) swallowed unknown flags. Reported by @dgrant — instinct was to check syntax with --help, got an install instead.

After

-h / --help print a usage banner and exit 0. The check runs before the bun availability guard so the flag is discoverable on a fresh machine that hasn't installed bun yet.

$ ./setup --help
gstack setup — install gstack skills + build browse binary

Usage: ./setup [options]

Options:
  --host <name>     Install for a specific host (claude, codex, kiro, factory,
                    opencode, openclaw, hermes, gbrain, auto). Default: claude.
  --prefix          Install skills with the gstack- prefix (e.g. /gstack-review).
  --no-prefix       Install skills with short names (e.g. /review). Default.
  --team            Switch to team mode (per-repo gstack with auto-update).
  --no-team         Force solo install even if a team-mode repo is detected.
  -q, --quiet       Suppress progress output.
  -h, --help        Show this help and exit.
...

Tests

test/setup-help.test.ts adds 5 cases following the static-pattern convention from test/setup-codesign.test.ts:

• usage function exists
• -h|--help is handled before the bun availability check
• usage text covers every documented flag
• ./setup --help exits 0, prints Usage:, runs no installer side effects
• ./setup -h is equivalent to ./setup --help

bun test test/setup-help.test.ts → 5 pass. Existing setup-codesign tests unaffected.

Scope notes

• Did not touch the catch-all *) shift ;; (silently swallows other unknown flags) — that's a separate behaviour worth its own discussion.
• No template/generated drift: the change is in setup only, no .tmpl regen needed.