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/view.rs

@@ -6,6 +6,7 @@ use clap::Args as ClapArgs;
 use serde_json::Value;
 
 use crate::client::Client;
+use crate::notify;
 
 #[derive(ClapArgs)]
 pub struct Args {
@@ -15,6 +16,9 @@ pub struct Args {
 
 pub fn run(client: &Client, args: Args) -> Result<()> {
     let repo = client.repo();
+    // 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);
     let issue = client.get_json(&format!("/repos/{repo}/issues/{}", args.number))?;
     let title = issue.get("title").and_then(Value::as_str).unwrap_or("");
     let body = issue.get("body").and_then(Value::as_str).unwrap_or("");