Add PyPI trusted publishing release workflow

[KKranthi6881]

Summary\n- add a tokenless PyPI Trusted Publishing release workflow using the GitHub pypi environment\n- document the maintainer release runbook and PyPI/GitHub one-time setup\n- update install docs and documentation tests for the release workflow\n\n## Verification\n- uv run --extra dev ruff check src tests\n- uv run --extra dev mypy src\n- uv run --extra dev pytest\n- uv run --extra dev --with build --with twine python -m build --sdist --wheel\n- uv run --extra dev --with twine python -m twine check dist/*\n- uv run --with ./dist/dbt_spec_kit-1.0.0-py3-none-any.whl dbt-specify --version\n\n## Release notes\nBefore the first PyPI publish, confirm PyPI has the pending Trusted Publisher for project dbt-spec-kit, owner duckcode-ai, repo dbt-spec-kit, workflow release.yml, environment pypi.