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/messages/agentMentions.ts

@@ -1,4 +1,4 @@
-import type { Agent } from "@opencode-ai/sdk";
+import type { Agent } from "@opencode-ai/sdk/v2";
 
 export interface AgentMentionSource {
   value: string;

packages/ui/src/lib/messages/messageText.ts

@@ -1,4 +1,4 @@
-import type { Part } from '@opencode-ai/sdk';
+import type { Part } from '@opencode-ai/sdk/v2';
 
 type TextLikePart = Part & { text?: string; content?: string };
 

packages/ui/src/lib/messages/synthetic.ts

@@ -1,4 +1,4 @@
-import type { Part } from "@opencode-ai/sdk";
+import type { Part } from "@opencode-ai/sdk/v2";
 
 export const isSyntheticPart = (part: Part | undefined): boolean => {
     if (!part || typeof part !== "object") {