Add release preflight checks for artifact smoke tests and changelog/version sync

[srichs]

Motivation

• Prevent releasing a package when pyproject.toml and CHANGELOG.md are out of sync and catch artifact/entrypoint problems before tagging.
• Provide an automated, reproducible preflight that builds wheel/sdist and verifies installed entrypoints to reduce release-time surprises.

Description

• Add a new module src/devr/release_preflight.py providing project_version, changelog_versions, validate_changelog, artifact build orchestration, and smoke_test_artifact that installs artifacts into isolated venvs and verifies devr --version and python -m devr --version.
• Add unit tests in tests/test_release_preflight.py covering changelog heading extraction and validation and project_version parsing.
• Update the release checklist in README.md to call python -m devr.release_preflight as the canonical preflight step and add an Unreleased changelog note documenting the change.

Testing

• Ran targeted tests with pytest -q tests/test_release_preflight.py tests/test_cli.py and they passed.
• Ran the full test suite with pytest -q and all tests passed (97 passed).
• Attempted to run the end-to-end preflight with PYTHONPATH=src python -m devr.release_preflight, but the environment lacks the build packaging module so the artifact build/smoke steps could not complete here (this is expected in the current sandbox).

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Codex Task