agent ui: open-threads section (questions + approvals pending)

new /api/open-threads endpoint on hive-ag3nt proxies the agent's
own GetOpenThreads RPC (manager flavour proxies the hive-wide
ManagerRequest::GetOpenThreads). same data the
mcp__hyperhive__get_open_threads tool sees from inside claude.

frontend renders a collapsible <details> section above the
terminal, listing each pending row (approval / question) with
asker → target, age, and free-form body. auto-expands on the
first appearance of any open thread; sticky after that.
refreshed on cold load + after every turn_end (turns are when
threads land or resolve).

hive-ag3nt/assets/index.html

@@ -29,6 +29,11 @@
     <ul id="inbox-list"></ul>
   </details>
 
+  <details id="open-threads-section" class="agent-inbox" hidden>
+    <summary>▸ <span id="open-threads-summary">open threads</span></summary>
+    <ul id="open-threads-list"></ul>
+  </details>
+
   <div class="terminal-wrap">
     <div id="live" class="live terminal"><div class="meta">connecting…</div></div>
     <div id="term-input" class="term-input"></div>