docs: rename SDD folders u6u-* → arcrun-* + update cross-refs

對應 SDD arcrun/.agents/specs/llm-interface/ M5.6（之前列為 backlog 連動）。
leo 2026-05-16 拍板 u6u 整體退場後，順手把兩個 SDD 也改名。

- .agents/specs/u6u-core-mvp/ → arcrun-core-mvp/
- .agents/specs/u6u-platform-evolution/ → arcrun-platform-evolution/
- CLAUDE.md / .claude/rules/00-sdd-protocol.md / 04-current-progress.md /
  llm-interface SDD 內所有跨 SDD 引用同步更新

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -58,8 +58,8 @@
 | Auth Recipe 系統 | `.agents/specs/arcrun/auth-recipe.md` |
 | Landing Page | `.agents/specs/arcrun/landing-page.md` |
 | SDK + Website | `.agents/specs/arcrun/sdk-and-website/` |
-| arcrun MVP 整體 | `.agents/specs/u6u-core-mvp/` |
-| u6u Platform Evolution | `.agents/specs/u6u-platform-evolution/` |
+| arcrun MVP 整體 | `.agents/specs/arcrun-core-mvp/` |
+| Platform Evolution | `.agents/specs/arcrun-platform-evolution/` |
 | Credential 長期規格（需求源） | `docs/user_requirements/credential_parts.md` |
 | Tech Stack 詳細 | `.agents/steerings/tech.md` |