Add release automation and is-upload input

[arcivanov]

Summary

• version_tool.py (new): AST-based tool for reading and modifying the version variable in PyBuilder build.py files. Stdlib only, compatible with Python 3.9-3.15+. Performs static analysis to prove version determinism, refusing to operate on files with dynamic versions, conditional assignments, exec/eval, star imports, or globals()/vars()/locals() calls.

• action.yml: New is-upload input replaces the ad-hoc PYB_EXTRA_ARGS=+upload shell step pattern. New release automation inputs (auto-release, auto-release-version, auto-github-release, auto-bump-dev, release-tag-prefix, release-title-template, build-py-path, github-token) with new steps for release detection, version stripping, atomic tag+commit push (distributed lock for multi-arch builds), GitHub Release creation, and dev version bump.

• test.yml: New test jobs for version_tool.py across Python 3.9/3.12/3.14, release detection, is-upload functionality, and end-to-end integration test. The integration test dispatches a workflow on pybuilder/build-integration-test that exercises the full release cycle on a real push event: detect [release], strip .dev, build, tag, GitHub release, and bump dev version.

• README.md: Updated advanced example, added Upload and Release Automation sections with migration guide. Updated actions/checkout and actions/setup-python version references from @v2 to @v5/@v6.

Test plan

• CI passes: existing test matrix (OS x Python x venv x homebrew)
• CI passes: test-version-tool across Python 3.9, 3.12, 3.14
• CI passes: test-release (release detection without push/upload)
• CI passes: test-is-upload (upload flag propagation)
• CI passes: integration-test (end-to-end release flow on pybuilder/build-integration-test)