design tool exits silently (SIGKILL 137) — reports DESIGN_READY but crashes on any subcommand

## What happened

Running `/plan-design-review` on keel (a Next.js personal finance app). The skill's DESIGN_READY check passed (binary exists at `~/.claude/skills/gstack/design/dist/design`, exec bit set). But every `design` subcommand crashes silently with exit code 137 (SIGKILL) and no output on stdout or stderr.

## Repro

```bash
D=~/.claude/skills/gstack/design/dist/design

# Binary exists and is executable
ls -la $D
# -rwxr-xr-x@ 1 user staff 61471936 Apr 15 09:19 /Users/.../design

# But any invocation crashes:
$D
# (no output, exit 0 but nothing happens)

$D --help
# (no output)

$D variants --help
# (no output, exit 137)

$D variants --brief "test" --count 1 --output-dir /tmp/design-test/
# (no output, exit 0, no files created in /tmp/design-test/ — directory never created)

$D setup
# (no output)
```

## Environment

- macOS 15.x (darwin 24.6.0), Apple Silicon
- gstack version `0.17.0.0` (per ~/.claude/skills/gstack/VERSION)
- `ANTHROPIC_API_KEY` set in env
- `config.yaml` has `telemetry: anonymous`, `proactive: true`, `auto_upgrade: true`

## Impact

`/plan-design-review` falls back to text-only mode as the skill spec allows, but the review is degraded — mockups are the primary way to evaluate design direction. Text description of UI is the fallback, not the target.

The silent failure mode is the frustrating part. If the binary required setup or a missing config, an error message would let me fix it. Exit 137 with no output suggests an OS-level kill (possibly a signing/quarantine issue on first run, or a Rosetta/arch mismatch).

## What I'd suggest

1. Print a useful error to stderr on startup if config/auth is missing
2. Handle the "first run" case gracefully (quarantine flag on fresh macOS download can trigger SIGKILL silently — `xattr -d com.apple.quarantine ` often fixes this)
3. `design --version` or `design --help` should never exit 137

## Context

Surfaced during `/plan-design-review` on keel's F122 feature (a ceremonial month-close screen). Review continued with text-based spec and completed successfully at 9/10, but the visual mockup step was the primary tool I wanted to use.

Happy to try `xattr -d com.apple.quarantine` or any other debug step and report back.