chore: clean up unused files

Removed:
- openteam binary from root (use bin/)
- opencatd.db (runtime artifact)
- dist/ (duplicate, already in backend/cmd/openteam/dist/)
- REFACTOR_PLAN.md (superseded by BACKEND_REFACTOR_PLAN.md)
- .pi/ (IDE artifact)

Updated .gitignore for new backend/ paths.

.gitignore

@@ -7,8 +7,8 @@ demo/
 openteam
 
 # 构建产物（make web 生成，由 go:embed 打进二进制）；保留 .gitkeep 占位使未构建前也能编译
-cmd/openteam/dist/*
-!cmd/openteam/dist/.gitkeep
+backend/cmd/openteam/dist/*
+!backend/cmd/openteam/dist/.gitkeep
 
 # 前端构建产物（项目根目录的副本）
 dist/