ci: automate versioning and changelog with commitizen

[gabrielfruet]

Derives the release version and CHANGELOG.md from Conventional Commit history, replacing the hand-edited version bump. Stacked on the enforcement PR (#2056), so merge that first.

Changes:

• [tool.commitizen] in pyproject.toml: standard SemVer mapping (fix→patch, feat→minor, ! or BREAKING CHANGE:→major). version_files keeps lightly.__version__ and the config version in sync. Not in any dependency group, so uv.lock is untouched.
• prepare_release.yml: manual workflow_dispatch running cz bump, dry-run by default. release_pypi.yml still publishes the tag it produces.
• CHANGELOG.md: seed file; the first real bump backfills it from history.

Before enabling non-dry-run:

• Pushing the bump commit and tag to master needs branch protection to allow the github-actions bot or a PAT.
• cz warns about two legacy tags (v.1.2.37, 1.1.8); ignored, but worth deleting.

Validation: cz bump --dry-run computed 1.5.26 → 1.6.0 (minor) from the feat: commits since v1.5.26; changelog grouped entries under Feat/Fix.

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