agent: use Effect schema for generated agent object

[kitlangton]

Summary

• Replace the remaining Zod object-generation schema in Agent.generate with an Effect Schema struct.
• Pass AI SDK a schema object containing both Standard Schema validation and Standard JSON Schema metadata.
• Preserve the generated object shape and leave provider/message behavior unchanged.

Verification

• bunx prettier --write packages/opencode/src/agent/agent.ts
• bun run lint -- packages/opencode/src/agent/agent.ts
• bun run test test/agent/agent.test.ts from packages/opencode
• bun typecheck from packages/opencode