docs: document patch dataset source

[LeiGuo622]

Summary

Type of change

• Backporting workflow
• Prejudge workflow
• Documentation
• Skill or agent guidance
• Build, packaging, or CI
• Tests or validation helpers
• Other

Affected workflow

• Main backporting flow (src/backporting.py, src/agent/, src/tools/)
• Prejudge flow (src/prejudge/)
• Runtime setup or configuration
• Documentation only
• Local Codex skill (skills/retropatch-engineering/)
• Not applicable

Validation

• python3 -m compileall -q src test
• python3 skills/retropatch-engineering/scripts/check_setup.py
• Targeted unit/helper script:
• Real backport/prejudge case:
• Not run; reason:

Safety notes

• This PR does not change destructive Git behavior.
• This PR does not require real API keys in committed files.
• This PR does not add private logs, patches, PoCs, or credentials.
• If an end-to-end run was performed, it used a disposable project_dir.

Checklist

• I kept the change scoped to the stated workflow.
• I updated documentation when behavior or user-facing workflow changed.
• I preserved existing compact prejudge result strings, or documented an intentional interface change.
• I avoided committing generated artifacts unless they are intentional examples.