docs: add AI-friendly visualization guidance

[jfust3]

Summary

• Adds llms.txt with AI-assistant guidance for Piccolo.jl visualization APIs
• Adds a quick-reference section to the visualization guide for AI-generated plotting code
• Documents common plotting helpers, required imports, and safe usage patterns
• Adds guardrails for animation outputs and Makie extension usage

Test Plan

• git diff --check origin/main...HEAD

Note: I did not run the Julia docs build in this environment because julia is not installed here.

Closes #61

[claude]

Claude Code Review

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[jack-champagne]

Hey @jfust3, thanks for your contribution. I think that the CONTEXT.md file is the file that contains most of what you've already submitted, perhaps just make changes to that instead of adding an llms.txt. Re-open if you do see changes to be incorporated in the CONTEXT.md file there. Thanks!

[jfust3]

Thanks — I moved the AI visualization guidance into src/visualizations/CONTEXT.md and removed the top-level llms.txt, keeping the existing visualization guide doc changes. I tried to reopen the PR but GitHub would not reopen it from this side; happy to reopen if permissions allow or resubmit if you prefer.

[jfust3]

Since GitHub would not let me reopen this closed PR, I opened the CONTEXT.md-based update here: #243.