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/verbs/comments.rs

@@ -21,6 +21,7 @@ use clap::Args as ClapArgs;
 use serde_json::{Value, json};
 
 use crate::client::Client;
+use crate::notify;
 use crate::verbs::print_json;
 
 /// Forgejo's per-page comment cap. The API caps `limit` at 50 even
@@ -51,6 +52,9 @@ pub fn run(client: &Client, args: Args) -> Result<()> {
         Some(n) => fetch_tail(client, repo, args.number, n)?,
         None => fetch_head(client, repo, args.number, args.limit)?,
     };
+    // Reading the thread clears its unread notification so the
+    // read-before-comment guard (in `comment`) lets a reply through.
+    notify::mark_read_best_effort(client, repo, args.number);
     if client.json_mode() {
         let trimmed: Vec<Value> = comments
             .iter()