refactor: slim code/ to pure code; consolidate experiments/ and docs

- Remove code/experiments/ → merge all eval JSONs into root experiments/
- Move code/exp.md, code/change.md → project root
- Delete code/2026-05-09-研究框架.md (duplicate of docs/)
- Update .gitignore: experiments/*.log (was code/experiments/*.log)
- Update code/CLAUDE.md: fix all affected paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -26,7 +26,7 @@ sync_v*.tar.gz
 sync_v*.zip
 
 # === 大型实验日志 ===
-code/experiments/*.log
+experiments/*.log
 
 # === 旧方向归档 ===
 旧方向信息/