Add artifact message persistence for session reconstruction

- Add ArtifactMessage type as core part of AppMessage union (not CustomMessages)
- ArtifactsRuntimeProvider appends artifact messages on create/update/delete
- MessageList filters out artifact messages (UI display only)
- artifacts.ts reconstructFromMessages handles artifact messages
- Export ARTIFACTS_RUNTIME_PROVIDER_DESCRIPTION from main index
- Fix artifact creation bug: pass filename as title instead of mimeType

Changes:
- web-ui/src/components/Messages.ts: Add ArtifactMessage to BaseMessage union
- web-ui/src/components/MessageList.ts: Skip artifact messages in render
- web-ui/src/components/sandbox/ArtifactsRuntimeProvider.ts: Append messages, fix title parameter
- web-ui/src/ChatPanel.ts: Pass agent.appendMessage callback
- web-ui/src/tools/artifacts/artifacts.ts: Handle artifact messages in reconstructFromMessages
- web-ui/src/index.ts: Export ARTIFACTS_RUNTIME_PROVIDER_DESCRIPTION
- web-ui/example/src/custom-messages.ts: Update message transformer to filter artifacts

packages/web-ui/example/src/custom-messages.ts

@@ -78,6 +78,11 @@ export function createSystemNotification(
 export function customMessageTransformer(messages: AppMessage[]): Message[] {
 	return messages
 		.filter((m) => {
+			// Filter out artifact messages - they're for session reconstruction only
+			if (m.role === "artifact") {
+				return false;
+			}
+
 			// Keep LLM-compatible messages + custom messages
 			return (
 				m.role === "user" ||