fix(core): restore v2 model prompts (#34275)

Co-authored-by: Aiden Cline <63023139+rekram1-node@users.noreply.github.com>
Co-authored-by: opencode-agent[bot] <opencode-agent[bot]@users.noreply.github.com>

packages/core/test/agent.test.ts

@@ -122,6 +122,7 @@ describe("AgentV2", () => {
         "summary",
         "title",
       ])
+      expect((yield* agent.get(AgentV2.defaultID))?.system).toBeUndefined()
       for (const item of agents) {
         expect(item.permissions.some((rule) => rule.action === "bash" && rule.effect !== "deny")).toBe(false)
       }