Docs: Update agent instructions about style classes (#5990)

## Summary

Very small change to help the LLMs follow the new patterns.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5990-Docs-Update-agent-instructions-about-style-classes-2876d73d365081339dbddf22d22947e7)
by [Unito](https://www.unito.io)

AGENTS.md

@@ -5,7 +5,7 @@
 - Routing/i18n/entry: `src/router.ts`, `src/i18n.ts`, `src/main.ts`.
 - Tests: unit/component in `tests-ui/` and `src/components/**/*.{test,spec}.ts`; E2E in `browser_tests/`.
 - Public assets: `public/`. Build output: `dist/`.
-- Config: `vite.config.mts`, `vitest.config.ts`, `playwright.config.ts`, `eslint.config.js`, `.prettierrc`.
+- Config: `vite.config.mts`, `vitest.config.ts`, `playwright.config.ts`, `eslint.config.ts`, `.prettierrc`.
 
 ## Build, Test, and Development Commands
 - `pnpm dev`: Start Vite dev server.

CLAUDE.md

@@ -126,6 +126,5 @@ const value = api.getServerFeature('config_name', defaultValue)  // Get config
 - NEVER use `--no-verify` flag when committing
 - NEVER delete or disable tests to make them pass
 - NEVER circumvent quality checks
-- NEVER use `dark:` prefix - always use `dark-theme:` for dark mode styles, for example: `dark-theme:text-white dark-theme:bg-black`
-- NEVER use `:class="[]"` to merge class names - always use `import { cn } from '@/utils/tailwindUtil'`, for example: `<div :class="cn('bg-red-500', { 'bg-blue-500': condition })" />`
-
+- NEVER use `dark:` or `dark-theme:` tailwind variants. Instead use a semantic value from the `style.css` theme, e.g. `bg-node-component-surface`
+- NEVER use `:class="[]"` to merge class names - always use `import { cn } from '@/utils/tailwindUtil'`, for example: `<div :class="cn('text-node-component-header-icon', hasError && 'text-danger')" />`