docs: update LLM guides for models and layout rules

Document new GUIDE.md expectations for src-layer edits and add index
entries for models and shared types. Clarify layout usage, composable
placement, error page conventions, and model/type ownership so future
changes follow the intended layer boundaries.docs: update LLM guides for models and layout rules

Document new GUIDE.md expectations for src-layer edits and add index
entries for models and shared types. Clarify layout usage, composable
placement, error page conventions, and model/type ownership so future
changes follow the intended layer boundaries.

src/composables/GUIDE.md

@@ -9,6 +9,8 @@
 - `layout/`：AppShell / layout 狀態與事件協調。
 - `maint/`：maintenance demo 的表單、CRUD、editable grid 狀態。
 
+頂層也可放通用 composable，例如 `useApiCall.ts`：封裝 loading / data / error / execute 模式、自動 snackbar 錯誤提示與取消請求過濾。適合為單一 API 呼叫提供輕量狀態管理，但不替代 page driver 或 command composable。
+
 ## 新增規則
 
 - 用 `useXxx.ts` 命名。