Add diagram skill with Mermaid, PlantUML, and Graphviz support

[ZihengXiong]

What changed

Added a new diagram skill with support for Mermaid, PlantUML, and Graphviz.

This includes:

• SKILL.md: entry point with tool-selection guide and quick-start flowchart
• mermaid.md: reference for diagram types, theming, and common pitfalls
• plantuml.md: examples for use case, component, deployment, activity, and state diagrams
• graphviz.md: DOT syntax, layout engines, clusters, and HTML labels
• scripts/render.py: unified renderer that auto-detects the input format

Why

This skill helps users generate and render diagrams more easily by providing structured guidance and examples for common diagram tools.

Testing

Checked that the new skill files are included and that the renderer supports Mermaid, PlantUML, and Graphviz inputs.

For PlantUML, the renderer falls back to the online API when plantuml.jar is not available.

[Copilot]

Pull request overview

Adds a new diagram skill to the skills library, providing reference documentation for Mermaid/PlantUML/Graphviz and a unified Python renderer script to convert diagram source files into PNG/SVG outputs.

Changes:

• Added skills/diagram/SKILL.md as the entry point with tool-selection guidance and rendering examples.
• Added large reference docs for Mermaid, PlantUML, and Graphviz to help users author diagram source.
• Added skills/diagram/scripts/render.py to auto-detect .mmd/.puml/.dot inputs and render via Mermaid CLI, local PlantUML jar / online PlantUML server fallback, or Graphviz.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
skills/diagram/SKILL.md	Skill entry point with selection guide and quick-start example.
skills/diagram/scripts/render.py	CLI renderer for Mermaid/PlantUML/Graphviz inputs.
skills/diagram/mermaid.md	Mermaid syntax reference and examples.
skills/diagram/plantuml.md	PlantUML reference and rendering guidance.
skills/diagram/graphviz.md	Graphviz DOT reference and rendering guidance.

Comments suppressed due to low confidence (2)

skills/diagram/plantuml.md:17

• Many fenced code blocks use a Windows file path as the info string (e.g., ```F:\supermarket...); Markdown will treat this as a language tag, causing incorrect rendering and no syntax highlighting. Replace these with appropriate fence languages (e.g., bash/plantuml/json/text) and remove machine-specific absolute paths throughout the document.

```F:\supermarket\skills\diagram\plantuml.md#L1-1
# 输出 PNG（默认）
java -jar plantuml.jar input.puml

# 输出 SVG

skills/diagram/graphviz.md:1092

• This fenced code block uses a Windows file path as the info string (```F:\supermarket...), which will render poorly in Markdown. Use an appropriate language tag (e.g., mermaid/text) and avoid machine-specific absolute paths.

```F:\supermarket\skills\diagram\graphviz.md#L1-1
flowchart LR
    A[用户请求] --> B{认证通过?}
    B -->|是| C[处理请求]
    B -->|否| D[返回 401]

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