Add --use-uv flag for faster venv creation using uv

[budaesandrei]

Summary

Adds an opt-in --use-uv flag to create_venvs.py that uses uv instead of the stdlib venv + pip for significantly faster virtual environment creation and package installation.

Changes

• Added is_uv_available() helper to check if uv is on PATH.
• Added uv_install() function that uses uv pip install --python <venv_python> for package installation.
• Updated create_venv() to accept a use_uv parameter:
  • When True: uses uv venv for venv creation and uv pip install for all packages, skipping the pip upgrade step (not needed with uv).
  • When False (default): behavior is completely unchanged.
• Added --use-uv argparse flag with a clear error message if uv is not installed.
• Updated README with usage example.

Compatibility

• CI workflows are unaffected — they call create_venvs.py --target production without --use-uv.
• uv venv produces a standard venv structure (bin/activate, bin/python) so all downstream consumers (quality checks, linting, unit tests, build scripts, VS Code settings) work identically.
• Requirements files use standard PEP 508 syntax fully supported by uv.
• The flag is purely opt-in — no existing behavior changes without it.

Testing

Verified the following scenarios:

• --use-uv with uv installed — creates venv with uv venv, installs packages with uv pip install
• Without --use-uv — identical behavior to before
• --use-uv without uv installed — exits with clear error message
• --use-uv with --recreate — properly deletes and recreates venv using uv
• --help — new flag appears in usage output