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/shared-memory/project-overview.md

@@ -51,6 +51,7 @@ server-rs + Axum + SpacetimeDB
 ## 当前文档入口
 
 - `docs/README.md`
+- `docs/【协作规范】Agent工作入口与执行准则-2026-06-22.md`
 - `docs/【项目基线】当前产品与工程约束-2026-05-15.md`
 - `docs/【后端架构】server-rs与SpacetimeDB数据契约-2026-05-15.md`
 - `docs/【玩法创作】平台入口与玩法链路-2026-05-15.md`