Add Agent execution guidelines & trim AGENTS.md

Introduce a new detailed Agent handbook (docs/【协作规范】Agent工作入口与执行准则-2026-06-22.md) and refactor AGENTS.md to be an entry-only navigation page. Update documentation cross-references (docs/README.md, project-memory README, document-map, team-conventions, development-workflow, decision-log and project-overview) to point to the new file and clarify reading/order rules, submission/validation constraints, and SpacetimeDB/back-end wording. Also adjust image canvas generation placement rules in the frontend doc and apply related changes to image-editor hooks/tests (src/components/image-editor/*). Purpose: separate high-level entry rules from complex execution details to reduce startup cost for complex tasks and centralize execution practices in a single guideline document.

docs/project-memory/README.md

@@ -21,7 +21,7 @@ docs/project-memory/
 
 ## 使用原则
 
-- 开发前先读 `AGENTS.md`，再按任务读取 `docs/project-memory/shared-memory/` 和当前 `docs/` 文档。
+- 开发前先读 `AGENTS.md`；复杂任务继续读取 `docs/【协作规范】Agent工作入口与执行准则-2026-06-22.md`，再按任务读取 `docs/project-memory/shared-memory/` 和当前 `docs/` 文档。
 - 长期有效的架构约定、接口变化、排障经验、开发流程和协作规则写入 `shared-memory/`。
 - 阶段性计划写入 `plans/`，已确定但暂未实施的共享 TODO 写入 `todos/`。
 - 如果本目录内容与代码或最新 `docs/` 冲突，以代码和最新 `docs/` 为准，并同步修正过期记忆。