Add optional model-bringup PR cleanup guidance

[hengtaoguo]

Description

Add an explicit opt-in cleanup phase, disabled by default, with a concise reference covering temporary tool removal, full-diff formatting, debugging cleanup, pylint fixes, and model-specific atomic PR splits. Delegate stack upload mechanics to upstream maxtext-pr-upload.

Tests

Using this subskill to split an example PR #5803 / #5804.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[gemini-code-assist]

Code Review

This pull request adds documentation and guidelines for large PR cleanups and atomic splits during model bring-up, introducing a new reference document pr_cleanup.md and updating SKILL.md. Feedback on the changes includes a correction to the documented Python line length limit, suggesting it be updated from 122 to 125 characters to match the repository's authoritative pylintrc configuration.