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.

No files, tests, or entry points are named. Start by mapping the existing session, participant, permission, update, comment, and notification flows, then define how view-only access, editing, presence, attribution, conflict handling, and notifications should work; done requires all listed collaboration behaviors to be specified and implemented safely.

Written by the indexing model from the issue text.