refactor: use core's chatPrefillFromUser hook instead of own click handler

PR #19 originally landed an entire user-list click handler inside
ep_ai_chat. That logic was generic — clicking a user in the user
list to prefill an @-mention is a discoverability win for any
multi-user pad, AI or no AI — and as @JohnMcLear pointed out, it
belongs in Etherpad core, not in this plugin.

The generic feature is now ether/etherpad#7660. This commit shrinks
ep_ai_chat's contribution to the only AI-specific bit: when the
clicked author is the AI, override the default "@AI_Assistant "
prefill with the configured trigger ("@ai " by default) so the
input lands in a useful state.

Implementation:
- Drop the in-plugin click delegation, the chatbox/chaticon DOM
  poking, and the input-prefill logic. Core handles all of that
  uniformly now.
- Add a chatPrefillFromUser client hook that returns the trigger
  string when the clicked authorId matches the AI's. The clientVars
  hook (added earlier in this PR) still exposes the trigger and
  authorId; nothing changes there.
- On older cores the new hook is never fired, so this is a no-op
  on a stock install — graceful degradation.

Net diff vs main is now: a tiny clientVars exposure on the server
and a ~10-line client hook handler. Everything else is core's job.

Author: JohnMcLear

ep.json

@@ -9,7 +9,8 @@
       },
       "client_hooks": {
         "postAceInit": "ep_ai_chat/static/js/index",
-        "chatSendMessage": "ep_ai_chat/static/js/index"
+        "chatSendMessage": "ep_ai_chat/static/js/index",
+        "chatPrefillFromUser": "ep_ai_chat/static/js/index"
       }
     }
   ]

static/js/index.js

@@ -9,78 +9,31 @@
 let padEditor = null;
 
 /**
- * postAceInit: store a reference to the ace editor for selection access,
- * and wire up the user-list click handler that prefills @mentions in
- * the chat input.
+ * postAceInit: store a reference to the ace editor for selection access.
  */
 exports.postAceInit = (hookName, context) => {
   padEditor = context.ace;
-  installUserListClickHandler();
 };
 
 /**
- * Wire up a click handler on the user list so clicking another editor
- * (or the AI Assistant) opens chat and prefills the input with an
- * @-mention of that author. Helps users discover the chat panel and
- * the @ai trigger without needing to be told "click Chat first."
+ * chatPrefillFromUser: when Etherpad core prefills "@<name> " in the chat
+ * input on a user-list click, swap in the configured trigger string for
+ * the AI's row. Without this, clicking the AI's chip in the user list
+ * would prefill "@AI_Assistant " which doesn't match anything the
+ * server-side mention extractor recognises.
  *
- * Implementation notes:
- *  - Uses a single delegated handler on document so it survives
- *    Etherpad re-rendering the user list table.
- *  - The AI's display name ("AI Assistant") would be a useless
- *    @-mention; clientVars.ep_ai_chat.{trigger,authorId} let us swap
- *    that in for the trigger string ("@ai") instead.
- *  - "Open chat" is a click on #chaticon — the same affordance the
- *    user would otherwise have to find themselves.
+ * Requires the chatPrefillFromUser hook (Etherpad >= the version that
+ * landed ether/etherpad#7660). On older cores this hook is silently
+ * never called and clicks fall back to the default prefill.
  */
-const installUserListClickHandler = () => {
-  document.addEventListener('click', (event) => {
-    try {
-      const tr = event.target.closest && event.target.closest('#otheruserstable tr');
-      if (!tr) return;
-      const authorId = tr.getAttribute('data-authorId');
-      if (!authorId) return;
-      // Skip clicks on the swatch (color-picker) — those have their own
-      // click semantics and shouldn't also prefill chat.
-      if (event.target.closest('.usertdswatch')) return;
-
-      const ai = (window.clientVars && window.clientVars.ep_ai_chat) || {};
-      const nameCell = tr.querySelector('.usertdname');
-      const nameText = nameCell ? (nameCell.textContent || '').trim() : '';
-
-      const mention = (ai.authorId && authorId === ai.authorId)
-        ? (ai.trigger || '@ai')
-        : `@${nameText.replace(/\s+/g, '_') || 'user'}`;
-
-      openChatAndPrefill(`${mention} `);
-      event.preventDefault();
-    } catch (e) { /* never break a click */ }
-  }, true);
-};
-
-const openChatAndPrefill = (text) => {
-  const chatBox = document.querySelector('#chatbox');
-  const visible = chatBox && chatBox.classList.contains('visible');
-  if (!visible) {
-    const chatIcon = document.querySelector('#chaticon');
-    if (chatIcon) chatIcon.click();
-  }
-  // Allow the chat panel to settle before focusing/prefilling.
-  setTimeout(() => {
-    const input = document.querySelector('#chatinput');
-    if (!input) return;
-    const current = input.value || '';
-    // Don't clobber what the user was typing. If the field is empty,
-    // or already starts with a stale @-token, replace it. Otherwise
-    // append.
-    if (!current.trim() || /^@\S+\s*$/.test(current.trim())) {
-      input.value = text;
-    } else if (!current.includes(text.trim())) {
-      input.value = `${current.trimEnd()} ${text}`;
+exports.chatPrefillFromUser = (hookName, context, cb) => {
+  try {
+    const ai = (window.clientVars && window.clientVars.ep_ai_chat) || {};
+    if (ai.authorId && context && context.authorId === ai.authorId) {
+      return cb(`${ai.trigger || '@ai'} `);
     }
-    input.focus();
-    try { input.setSelectionRange(input.value.length, input.value.length); } catch (_e) { /* */ }
-  }, 50);
+  } catch (_e) { /* fall through to default */ }
+  return cb();
 };
 
 /**