feat(markdown): render Pikchr diagrams

[matt2e]

Adds shared Pikchr diagram rendering for markdown, wires note/session prompts to include Pikchr grammar guidance, and covers diagram formatting/rendering with tests.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: ef9ab9d125

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[chatgpt-codex-connector]

Preserve Pikchr dimensions when rendering previews

When pikchr-js returns an SVG without explicit width/height attributes (the fixture in these tests is one such SVG), this path discards the renderedPikchr.width/height values and renders only the sanitized SVG. Inline SVGs without dimensions fall back to browser default sizing, so small or non-default-size Pikchr diagrams in notes/sessions render at the wrong size instead of the dimensions computed by the renderer; pass those dimensions through or inject them into the sanitized SVG.

Useful? React with 👍 / 👎.