feat(room): render AI mentions as 🤖 button with click-to-insert action

AI mentions now render as "🤖 AI: name" buttons instead of plain text.
Clicking a 🤖 button inserts the @ai:mention into the message input at
the cursor position, enabling users to summon that AI model into the conversation.

Implementation:
- MessageMentions renders AI mentions as styled buttons with 🤖 icon
- Click dispatches 'mention-click' CustomEvent on document
- ChatInputArea listens and inserts the mention HTML at cursor

src/components/room/MessageMentions.tsx

@@ -127,8 +127,40 @@ function renderNode(
   }
   if (node.type === 'mention') {
     const displayName = resolveName(node.mentionType, node.id, node.label);
+    const baseClass = mentionStyles[node.mentionType] ?? mentionStyles.user;
+
+    if (node.mentionType === 'ai') {
+      return (
+        <button
+          key={index}
+          type="button"
+          className={cn(
+            baseClass,
+            'inline-flex items-center gap-1 cursor-pointer border-0 bg-transparent p-0',
+            'hover:opacity-80 transition-opacity',
+          )}
+          onClick={() => {
+            document.dispatchEvent(
+              new CustomEvent('mention-click', {
+                detail: {
+                  type: 'ai',
+                  id: node.id,
+                  label: displayName,
+                },
+                bubbles: true,
+              }),
+            );
+          }}
+        >
+          <span className="text-sm">🤖</span>
+          <span>AI:</span>
+          <span className="font-medium">{displayName}</span>
+        </button>
+      );
+    }
+
     return (
-      <span key={index} className={mentionStyles[node.mentionType] ?? mentionStyles.user}>
+      <span key={index} className={baseClass}>
         @{displayName}
       </span>
     );

src/components/room/RoomChatPanel.tsx

@@ -230,6 +230,35 @@ const ChatInputArea = memo(function ChatInputArea({
     }
   };
 
+  // Listen for mention-click events from message content (e.g. 🤖 AI button)
+  useEffect(() => {
+    const onMentionClick = (e: Event) => {
+      const { type, id, label } = (e as CustomEvent<{ type: string; id: string; label: string }>).detail;
+      if (!textareaRef.current) return;
+      const textarea = textareaRef.current;
+      const cursorPos = textarea.selectionStart;
+      const textBefore = textarea.value.substring(0, cursorPos);
+
+      if (type === 'ai') {
+        // Insert @ai:mention at cursor position
+        const html = buildMentionHtml('ai', id, label);
+        const spacer = ' ';
+        const newValue = textBefore + html + spacer + textarea.value.substring(cursorPos);
+        const newCursorPos = cursorPos + html.length + spacer.length;
+        onDraftChange(newValue);
+        setTimeout(() => {
+          if (textareaRef.current) {
+            textareaRef.current.value = newValue;
+            textareaRef.current.setSelectionRange(newCursorPos, newCursorPos);
+            textareaRef.current.focus();
+          }
+        }, 0);
+      }
+    };
+    document.addEventListener('mention-click', onMentionClick);
+    return () => document.removeEventListener('mention-click', onMentionClick);
+  }, []);
+
   return (
     <div className="border-t border-border/70 bg-background p-3">
       {replyingTo && (