feat(bump): add --version-files-only and deprecate --files-only

[2509abhi]

This PR implements the changes requested in #1658.

Implements the historical recommendations described in #1640 (comment).

New Feature

• Added a new CLI flag: --version-files-only
• Updates only the files in version_files without creating a commit or tag

Deprecation

• Marked --files-only as deprecated (to be removed in v5)
• Added CLI and docs deprecation notices

Configuration Handling

• Supported legacy config key files_only if present
• Ensured backward compatibility until v5

Documentation Updates

• Updated bump.md to document --version-files-only
• Added deprecation note for --files-only
• Added historical note about old files → version_files

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Closes #1658

[bearomorphism]

pls run though our pr checklist (you should see it upon PR creation) to pass the PR pipeline.

just quickly scanned through your changes, looks like you didn't run poetry lint or poetry format

[Lee-W]

All commit messages must meet our requirements. Run poetry all locally to identify any issues. Thanks :)