feat(web): /skill slash command with autocomplete

Trigger /<name>, dropdown lists all skills filtered by name prefix,
arg passthrough sends the rest as the user message. Synthetic
skill_use tool_use renders identically to model-invoked skills.

apps/web/src/components/panes/ChatPane.tsx

@@ -96,6 +96,18 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
     }
   }, [chatId]);
 
+  // Batch 9.6: slash-command dispatch. Sent regardless of streaming state —
+  // matches the existing /compact precedent (which also fires immediately).
+  // Empty args go to the server as null; the server fills in a default user
+  // message ("Apply this skill.") so the model has something to act on.
+  const handleSlashCommand = useCallback(async (skillName: string, userMessage: string) => {
+    try {
+      await api.chats.skillInvoke(chatId, skillName, userMessage.length > 0 ? userMessage : null);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : `/${skillName} failed`);
+    }
+  }, [chatId]);
+
   function removeQueued(idx: number) {
     setQueue((prev) => prev.filter((_, i) => i !== idx));
   }
@@ -183,6 +195,7 @@ export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange,
           webSearchEnabled={webSearchEnabled}
           onSend={handleSend}
           onForceSend={streaming ? handleForceSend : undefined}
+          onSlashCommand={handleSlashCommand}
         />
       </div>
     </div>