docs(ui): add AGENTS.md files for component documentation

src/styles/AGENTS.md

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+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# styles
+
+## Purpose
+Global CSS and component/block styles. Tailwind v4 is configured via CSS (`global.css`) rather than a large JS config.
+
+## Key Files
+| File | Description |
+|------|-------------|
+| `global.css` | Tailwind import + theme: color tokens (`--color-global-bg`, `--color-global-text`, `--color-accent`, `--color-link`, `--color-muted`, `--color-special-*`), dark/light variants, base element styles, and the `cactus-link` / `title` utility classes. |
+
+## Subdirectories
+| Directory | Purpose |
+|-----------|---------|
+| `blocks/` | `search.css` — Pagefind search UI styling. |
+| `components/` | `admonition.css` and `github-card.css` — styles for the matching remark plugins. |
+
+## For AI Agents
+
+### Working In This Directory
+- Use the defined color tokens (e.g. `text-muted`, `bg-global-bg`, `border-global-text/15`) for theme consistency rather than hardcoded colors — they adapt to dark/light mode.
+- New markdown block plugins (in `src/plugins/`) should get their CSS here under `components/`.
+
+## Dependencies
+
+### External
+- `tailwindcss` v4, `@tailwindcss/typography` (prose), `cssnano`, `autoprefixer`.
+
+<!-- MANUAL: -->