Add MaxText automated release preparation skill

[igorts-git]

Description

This PR introduces a standardized AI agent skill file (SKILL.md) designed to orchestrate the end-to-end validation and release preparation lifecycle for MaxText PyPI packages.
The workflow automates tedious verification matrices while enforcing strict zero-regression safety boundaries before prompting a maintainer for final publication authorization.

Key Workflow Constraints & Features

• Strictly Read-Only Validation: Prohibits automated source code patches during execution. If any test returns unexpected stderr, a stack trace, or a non-zero exit, the agent halts and reports verbatim diagnostics.
• Clean State Enforcement: Enforces fresh virtual environments (uv venv) for isolated phases and validates clean git tree boundaries to prevent stale artifact pollution.
• Phased Execution Matrices:
  • Phase 1: Version verification (src/maxtext/__init__.py) and PyPI wheel compilation.
  • Phase 2: Single-host TPU validation (Pre-training, Post-training SFT/GRPO, and vLLM inference decodes).
  • Phase 3: Multi-host cluster orchestration (Image builds, McJAX/Pathways XPK job scheduling, checkpoint persistence checks).
  • Phase 4: Source checkout integrity builds and benchmark runner validation matrices (Stable vs. Nightly).
• Maintainer Ownership: Generates a final executive report matrix detailing successful logs vs. anomaly snippets to support the final human GO/NO-GO release decision.

Tests

Validation steps:

• I created a new TPU v5 VM.
  • Installed required OS patches
  • Configured docker
  • Installed gemini-cli
  • Cloned this maxtext branch.
• Opened "gemini" in the maxtext directory, signed in, and prompted it: "please run the release process"
• Gemini-CLI asked me a few questions (HF_TOKEN, XPK cluster name, etc)
• I clicked Ctrl+Y to enable the YOLO mode. It ran for a few hours processing all validation steps.

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.