v0.4.0 — Execution runners (Docker, Python-uv) and documentation overhaul

- Docker runner: run tasks inside any container image
- Python-uv runner: write inline Python scripts directly in bam.yaml
- Runner-aware cache keys (runner type + image included)
- README links fixed for PyPI (all relative links → absolute GitLab URLs)
- Release workflow documented in AGENTS.md

See CHANGELOG.md for the full list of changes.