Add CLAUDE.md and PLAN.md

[thchr]

Summary

• Adds CLAUDE.md with API summary, project structure, conventions, known issues, and developer preferences
• Adds PLAN.md with a phased improvement plan covering: TODO audit, devdocs restructuring, tests & coverage, refactoring, and symmetry analysis correctness

Context

Starting point for a systematic round of maintenance, testing, and bug-fixing work. The plan is ordered to avoid regressions (tests before refactoring; refactoring before the tricky symmetry analysis fix).

🤖 Generated with Claude Code

[thchr]

Hi @AntonioMoralesPerez: this is just me exploring how much stuff we can get done by leaning on Claude's Opus model - we'll see how productive it is.

I suspect it won't be able to solve our issues in #89, but it's very good at getting things phrased systematically - so if nothing else, it will do that for it. I'll also try to use it for some general clean-up of everything and for any minor things that arise. I'll PR everything in separate bundles - probably will be over some days (eats tokens so fast 😭)...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[thchr]

Ah, I will ask explicitly: are you okay with me merging this? No functional changes, just a CLAUDE.md file for future use and a tentative plan for where to focus AI efforts.

[AntonioMoralesPerez]

Hi @thchr, sure thing, go ahead! It is just two .md files, no problem. Later today I might add my thoughts and concerns on them as another PR.