Share / Collaboratively work together with colleagues in a session

Allow users to share a session with colleagues so multiple people can view, discuss, and contribute to the same work in real time. Include permission controls such as view-only and edit access, visible participant presence, synchronized updates, comments, and notifications. Changes should be attributed to each contributor and safely handled when multiple users edit simultaneously.

未指定任何文件、测试或入口点。首先梳理现有的会话、参与者、权限、更新、评论和通知流程，然后定义只读访问、编辑、在线状态、归属、冲突处理和通知应如何工作；只有在所有列出的协作行为都已完成规范定义并安全实现后，才算完成。

由索引模型根据 Issue 内容生成。