Add Codex PR diagram skill

[parsakhaz]

Summary

Adds a user-level Codex skill for creating PR visual diagrams and updates the Codex prepare-pr workflow so future PR descriptions include a ## Visual Overview by default.

Why

We want visual PR reviews to be part of the normal workflow, not a one-off local tweak. The previous setup only existed in ~/.codex/skills, so it could be overwritten or lost when user-level skills are restored from this repo.

Before

flowchart LR
  PR[Prepare PR] --> Body[Text-only PR body]
  Local[Local Excalidraw tweak] -.not persisted.-> Lost[Can be overwritten]

Loading

After

flowchart LR
  PR[Prepare PR] --> Diagram[Create visual overview]
  Diagram --> Tmp[Work files under /tmp/codex-pr-diagrams]
  Diagram --> Mermaid[GitHub-rendered Mermaid in PR body]
  Skill[dcouple/skills] --> Restore[User-level Codex skill survives reinstall]

Loading

What changed

• Adds parsa/.codex/skills/excalidraw-pr-diagrams.
• Pins the Excalidraw renderer import to @excalidraw/excalidraw@0.18.0 because the unpinned current bundle resolves to a broken transitive ESM import in headless Chromium.
• Updates parsa/.codex/skills/prepare-pr/SKILL.md to include PR visual overview creation.
• Requires generated diagram work to stay in /tmp, normally /tmp/codex-pr-diagrams/<branch-or-pr>/.

Verification

• Ran uv sync in the copied skill renderer directory.
• Rendered an existing .excalidraw file to PNG using the copied skill from this repo.
• Confirmed generated artifacts stay outside app repos under /tmp/codex-pr-diagrams.