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/client.rs

@@ -229,6 +229,22 @@ impl Client {
         Ok(())
     }
 
+    /// PATCH `<api>/<path>` for an endpoint that returns a 2xx with an
+    /// empty body (nothing to decode). Used to mark a notification thread
+    /// read (`/notifications/threads/{id}` answers `205 Reset Content`).
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the request fails to send (transport/network
+    /// error) or the server responds with a non-2xx status (the response
+    /// body is included in the error).
+    pub fn patch_no_content(&self, path: &str) -> Result<()> {
+        let url = format!("{}{}", self.api(), path);
+        let resp = self.http.patch(&url).send().context("PATCH")?;
+        check_status(resp, &format!("PATCH {url}"))?;
+        Ok(())
+    }
+
     /// POST a JSON body to `<api>/<path>` for an endpoint that returns a
     /// 2xx with an empty body (so there is nothing to decode). Used by
     /// `pr-merge` — Forgejo's merge endpoint answers `200 OK` with no body