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/README.md

@@ -1,9 +1,10 @@
 # 文档总览
 
-`docs/` 现在按主题拆成了 6 类；旧后端路线文档开始聚合和删除，后续实现以 Rust / SpacetimeDB 当前基线为准。
+`docs/` 现在按主题维护项目当前口径；旧后端路线文档开始聚合和删除，后续实现以 Rust / SpacetimeDB 当前基线为准。
 
 ## 快速入口
 
+- [Agent 工作入口与执行准则](./%E3%80%90%E5%8D%8F%E4%BD%9C%E8%A7%84%E8%8C%83%E3%80%91Agent%E5%B7%A5%E4%BD%9C%E5%85%A5%E5%8F%A3%E4%B8%8E%E6%89%A7%E8%A1%8C%E5%87%86%E5%88%99-2026-06-22.md)：复杂任务前的 Agent 阅读顺序、执行边界、技能路由、文档规则和验证口径。
 - [经验沉淀](./experience/README.md)：项目开发经验、UI 交接、历史实现经验。
 - [审计与复盘](./audits/README.md)：工程审查、文本/乱码审计、专项落地审计。
 - [系统设计](./design/README.md)：玩法、关系、物品与对话设计。