Rebuild on forge's own notification read-state instead of the local
seen-cursor mirror (operator-nacked: mirrors state forge owns + drifts
on restart).
- comment refuses to post when forge has an unread notification for the
thread (someone commented since you last read it); --force overrides.
Degrades open if the notification check itself fails.
- comments / view mark the thread's notification read (the "I've seen
it" signal), clearing the guard for a subsequent reply.
- new crate::notify (no local file): unread_thread_id pages repo-scoped
unread notifications (newest-first, capped) + subject-url number match;
mark_thread_read via the new client.patch_no_content.
Pairs with the forge_notify harness change (#1895, merged) that leaves
delivered notifications unread until the agent actually reads. Covers
pr/issue comment too (they delegate to comment::run).
156ca70b1e