feat(#1888): read-before-comment guard via forge notification read-state

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).

hive-forge/src/main.rs

@@ -18,6 +18,7 @@
 
 mod body;
 mod client;
+mod notify;
 mod verbs;
 
 use anyhow::{Context, Result};