- Add onBeforeSend callback to ChatPanel and AgentInterface
- Add onBeforeToolCall callback (for future permission dialogs)
- Create NavigationMessage custom message type
- Add browserMessageTransformer that converts nav messages to <system> tags
- Track last submitted URL and tab index
- Auto-insert navigation message when URL/tab changes on user submit
- Navigation message shows favicon + page title in UI
- LLM receives: <system>Navigated to [title] (tab X): [url]</system>
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