Convert project packaging and workflows from Poetry to uv

[scaringi]

Summary

• replace Poetry-managed packaging metadata with a standard pyproject configured for uv
• convert optional llm and keyring installs into explicit extras and keep dev tooling in a dependency group
• add .python-version and uv.lock, and update README workflows to use uv sync and uv run

Validation

• uv lock
• uv sync --group dev --extra llm --extra keyring
• uv run pytest -v
• uv run ruff check src/ tests/
• uv build

[pwerry]

This is a heavy change - let's democratize it and gather some upvotes!