Move diagramming instructions into skill

[ammario]

We currently insert a <mermaid>...</mermaid> instruction block into every request, even though only a few of them will ever render a mermaid diagram. This behavior is a vestige of our pre-skills approach to context.

We should:

• Move the mermaid instructions into a "diagram" built-in skill (🤖 feat: move Mermaid instructions into built-in diagram skill #2611)
• Perhaps take the opportunity to expand on the instructions and add support other charting methods