fix: harden v0.3.1 dual release workflow

[HERRY423]

Summary

• replace shell-dependent checksum redirections with a tested cross-platform Python builder
• fail closed unless the current version has exactly one wheel, one sdist, and one Agent plugin bundle
• bump backend and plugin metadata to 0.3.1 and document that v0.3.0 created neither a GitHub Release nor a PyPI distribution
• keep release claims limited to packaging infrastructure; no biological validation claim is added

Local verification

• ruff check src/ tests/ scripts/build_plugin_bundle.py scripts/build_release_checksums.py
• ruff format --check src/ tests/ scripts/build_plugin_bundle.py scripts/build_release_checksums.py
• pytest tests/test_release_packaging.py: 6 passed
• built the 0.3.1 wheel, sdist, Agent plugin bundle, and deterministic SHA256SUMS
• installed both the wheel and unpacked plugin bundle in separate virtual environments
• celltypepilot --version and celltypepilot doctor --json passed from the installed wheel

The local host lacks Twine and cannot download isolated build dependencies inside its network sandbox. The workflow installs those release dependencies explicitly; full cross-version tests and Twine verification are therefore required from CI before merge.