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/components/message-renderer-registry.ts

@@ -0,0 +1,28 @@
+import type { TemplateResult } from "lit";
+import type { AppMessage } from "./Messages.js";
+
+// Extract role type from AppMessage union
+export type MessageRole = AppMessage["role"];
+
+// Generic message renderer typed to specific message type
+export interface MessageRenderer<TMessage extends AppMessage = AppMessage> {
+	render(message: TMessage): TemplateResult;
+}
+
+// Registry of custom message renderers by role
+const messageRenderers = new Map<MessageRole, MessageRenderer<any>>();
+
+export function registerMessageRenderer<TRole extends MessageRole>(
+	role: TRole,
+	renderer: MessageRenderer<Extract<AppMessage, { role: TRole }>>,
+): void {
+	messageRenderers.set(role, renderer);
+}
+
+export function getMessageRenderer(role: MessageRole): MessageRenderer | undefined {
+	return messageRenderers.get(role);
+}
+
+export function renderMessage(message: AppMessage): TemplateResult | undefined {
+	return messageRenderers.get(message.role)?.render(message);
+}