fix(core): request default OpenAI reasoning summaries

[opencode-agent]

Summary

• apply OpenAI Responses defaults when V2 resolves catalog-backed @ai-sdk/openai models
• request reasoning.summary: "auto" for default GPT-5 reasoning models, not only explicit variants
• retain stateless encrypted reasoning defaults through the shared OpenAI option normalizer

Root cause

V2 maps catalog-backed OpenAI models directly onto the native Responses route but skipped the OpenAI model default normalizer. Therefore the normal/default GPT-5 selection sent no reasoning.summary request. Explicit reasoning variants happened to add reasoningSummary: "auto", but default model use produced no visible summary events for the TUI.

Checks

• bun test test/session-runner-model.test.ts (22 pass)
• bun typecheck in packages/core
• bun typecheck in packages/llm

[opencode-agent]

Closing after clarification: OpenAI is returning the available reasoning summary headline; V2 is not dropping a fuller thought payload. The proposed change incorrectly treated provider behavior as a product bug.