docs: add Claude Design system prompt

Source: elder-plinius/CL4R1T4S (ANTHROPIC/Claude-Design-Sys-Prompt.txt).
The prompt governs Claude's Design feature for producing HTML artifacts
(decks, prototypes, animations), not Claude Code CLI.

https://claude.ai/code/session_01Mm5cao4DWuqXogHPaccCTy

Author: claude

prompts/official-product/TOC.md

@@ -18,6 +18,7 @@
 - [ClaudeDesktopSystemPrompt](./claude/ClaudeDesktopSystemPrompt.md)
 - [ClaudeSonnet4](./claude/ClaudeSonnet4.md)
 - [ClaudeWithCode](./claude/ClaudeWithCode.md)
+- [ClaudeDesign](./claude/ClaudeDesign.md)
 - [README](./claude/README.md)
 - [tools](./claude/tools.md)