Add auto-merge and automatic releases

[aviadr1]

Summary

• add Dependabot auto-merge after the CI workflow succeeds
• add automatic version-tag creation from pyproject.toml on main and dispatch the release workflow for that tag
• keep tag-based/manual release support and create GitHub Releases with built artifacts
• add real backend smoke tests so CI validates uvloop on POSIX and winloop on Windows
• strengthen README and contributing docs around upstream vs winuvloop, compatibility, CI, Dependabot, and release automation

Validation

• uv lock --check
• uv run ruff check .
• uv run pytest
• uv build
• uv run twine check dist/*
• parsed workflow and Dependabot YAML with PyYAML