feat: migrate to OpenCode SDK v2

Update all import statements to use @opencode-ai/sdk/v2
Modify Vite configurations to alias new SDK path
Update API client method signatures for SDK v2 compatibility

packages/ui/src/lib/messageFreshness.ts

@@ -1,4 +1,4 @@
-import type { Message } from '@opencode-ai/sdk';
+import type { Message } from '@opencode-ai/sdk/v2';
 
 export class MessageFreshnessDetector {
     private static instance: MessageFreshnessDetector;