fix(metadata): update default model to 'opencode/big-pickle'

Author: moazbuilds

src/infra/engines/providers/opencode/metadata.ts

@@ -7,6 +7,6 @@ export const metadata: EngineMetadata = {
   cliCommand: 'opencode',
   cliBinary: 'opencode',
   installCommand: 'npm i -g opencode-ai@latest',
-  defaultModel: 'opencode/glm-4.7-free',
+  defaultModel: 'opencode/big-pickle',
   order: 1,
 };