ci: Improve model regeneration workflow (#709)

## Summary

- Add reviewer, assignee, and label to auto-created PRs
- Add `@docs_group('Models')` post-processing step for API docs
- Replace force push with incremental commits to preserve reviewer changes
- Simplify PR description body

### Issues

- Closes: #642

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>