Docs: Slim down AGENTS.md, add commands

- Restructure `AGENTS.md` from 245 to 93 lines — keep only what every agent needs on every task (file structure, testing commands, debugging pointers, critical rules, workflow pointers)
- Move TS/Svelte/Rust/CSS code rules and design guidelines to `style-guide.md` where they belong
- Extract wrap-up checklist and planning workflow into new `@wrap-up` and `@plan` Claude commands (invoked on demand instead of read every time)
- Enhance `@commit-draft` command with inline format rules
- Deduplicate git, JSDoc, and CSS rules that appeared in both files

Author: vdavid