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/messageCompletion.ts

@@ -1,5 +1,5 @@
 /* eslint-disable @typescript-eslint/no-explicit-any */
-import type { Part } from "@opencode-ai/sdk";
+import type { Part } from "@opencode-ai/sdk/v2";
 import { isFullySyntheticMessage } from "@/lib/messages/synthetic";
 
 export interface MessageInfo {