feat(skills): add excalidraw-diagram-generator skill and docs update

[jun-shiromizu]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, or skill file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, or skill with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

Summary

Add a new skill, excalidraw-diagram-generator, to generate Excalidraw .excalidraw JSON diagrams from natural language requests. It supports multiple diagram types (flowcharts, relationship diagrams, mind maps, architecture, DFD, swimlane, class, sequence, ER) and provides step-by-step guidance and templates. Includes guidance for optional icon library usage via bundled scripts.

Why / Use cases

• Create diagrams directly from Copilot prompts without manual drawing
• Quickly visualize workflows, system architecture, data flows, and entity relations
• Produce reusable .excalidraw files that open in Excalidraw or the VS Code extension

How to use (quick start)

1. Use the skill when asked to “create a diagram / flowchart / architecture / mind map”.
2. Provide a clear description of key entities, steps, and relationships.
3. Output a .excalidraw JSON file and open it in Excalidraw (web or VS Code extension).

Validation / Tests

• npm run skill:validate
• npm run build
• bash scripts/fix-line-endings.sh

Distinct from existing skills

• Focuses specifically on generating Excalidraw-format diagram files (not Mermaid/PlantUML/ASCII).
• Includes templates, scripts, and guidance for diagram construction and optional icon library workflows.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New collection file.
• New skill file.
• Update to existing instruction, prompt, agent, collection or skill.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.