chore: align mvp design governance

- 统一 local-first MVP 后阶段架构口径，补充 process 执行总序和 Reasonix 协作记录

- 归档已完成的 design checklist，标注参考型 process，更新 AGENTS/REASONIX/架构文档

- 补充文件树/主编辑器下载与上下文菜单相关实现、bug 记录和 smoke 脚本

验证：git diff --check；codegraph sync .；cargo test -p mnote-web；node --check scripts/task476-filetree-editor-context-menu-download-smoke.js

.codegraph/.gitignore

@@ -0,0 +1,16 @@
+# CodeGraph data files
+# These are local to each machine and should not be committed
+
+# Database
+*.db
+*.db-wal
+*.db-shm
+
+# Cache
+cache/
+
+# Logs
+*.log
+
+# Hook markers
+.dirty