Add GitHub release pipeline

[jmcte]

Summary

• add a tag-triggered GitHub Release workflow that validates the tag, runs fast checks, builds sdist/wheel, writes SHA256 sums, extracts the matching changelog section, and publishes release assets
• add scripts/release/dry-run.sh for pre-tag build verification
• add a PR Fast CI package-build job under the required CI Gate for release/packaging changes
• flip project.bootstrap.yaml release capability to github-release

Verification

• bash -n scripts/release/dry-run.sh
• bash scripts/ci/check-ci-contract.sh
• PYTHON_BIN=/private/tmp/mailplus-release-venv/bin/python bash scripts/release/dry-run.sh --skip-fast-checks
• bash scripts/ci/run-fast-checks.sh

Closes #86.

[chatgpt-codex-connector]

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