This release improves the first-run experience by making Python packages
auto-install during both mix compile and mix test.

Changes:
- Default auto_install changed from :dev to :dev_test
- Added :dev_test option matching both dev and test environments
- Environment variable SNAKEBRIDGE_AUTO_INSTALL now accepts dev_test value
- Updated all examples to use the new default

Fixed:
- mix test now works out of the box without requiring mix snakebridge.setup

Documentation:
- Added uv installation instructions to README
- Added required compiler configuration to quick start example
- Clarified snakebridge compiler requirement for auto-install

Dependencies:
- Bump snakepit from 0.8.8 to 0.8.9