- Track agent busy state: isStreaming OR pendingToolCalls.size > 0
- When agent transitions from busy to idle, check for URL changes
- Extract checkAndInsertNavMessage() helper function
- Call helper from both onBeforeSend (user submit) and agent state subscription (post-tool)
- Fixes issue where browser_javascript tool navigates page but no nav message inserted
- Navigation messages now appear after tools that change window.location.href