This PR adds a standalone script to download pre-built CI artifacts from
GitHub Actions. This allows us to quickly switch commits without
rebuilding.
**Features:**
- Downloads artifacts for current HEAD or specified commit (`--sha`)
- Caches in `~/.cache/lean_build_artifact/` for reuse
- Platform detection (Linux/macOS, x86_64/aarch64)
**Usage:**
```
build_artifact.py # Download for current HEAD
build_artifact.py --sha abc1234 # Download for specific commit
build_artifact.py --clear-cache # Clear cache
```
This is extracted to be shared with `lean-bisect`.
🤖 Prepared with Claude Code
Co-authored-by: Claude <noreply@anthropic.com>
