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/agent/transports/ProviderTransport.ts

@@ -27,18 +27,10 @@ export class ProviderTransport implements AgentTransport {
 			};
 		}
 
-		// Filter out attachments from messages
-		const filteredMessages = messages.map((m) => {
-			if (m.role === "user") {
-				const { attachments, ...rest } = m as any;
-				return rest;
-			}
-			return m;
-		});
-
+		// Messages are already LLM-compatible (filtered by Agent)
 		const context: AgentContext = {
 			systemPrompt: cfg.systemPrompt,
-			messages: filteredMessages,
+			messages,
 			tools: cfg.tools,
 		};