Add custom message extension system with typed renderers and message transformer

- Implement CustomMessages interface for type-safe message extension via declaration merging
- Add MessageRenderer<T> with generic typing for custom message rendering
- Add messageTransformer to Agent for filtering/transforming messages before LLM
- Move message filtering from transports to Agent (separation of concerns)
- Add message renderer registry with typed role support
- Update web-ui example with SystemNotificationMessage demo
- Add custom transformer that converts notifications to <system> tags
- Add SessionListDialog onDelete callback for active session cleanup
- Handle non-existent session IDs in URL (redirect to new session)
- Update both web-ui example and browser extension with session fixes

packages/web-ui/src/index.ts

@@ -17,8 +17,16 @@ export { Input } from "./components/Input.js";
 export { MessageEditor } from "./components/MessageEditor.js";
 export { MessageList } from "./components/MessageList.js";
 // Message components
-export type { AppMessage } from "./components/Messages.js";
+export type { AppMessage, CustomMessages } from "./components/Messages.js";
 export { AssistantMessage, ToolMessage, UserMessage } from "./components/Messages.js";
+// Message renderer registry
+export {
+	getMessageRenderer,
+	type MessageRenderer,
+	type MessageRole,
+	registerMessageRenderer,
+	renderMessage,
+} from "./components/message-renderer-registry.js";
 export {
 	type SandboxFile,
 	SandboxIframe,