Make Pro empty-response recovery reliable by default

[cinnabarhorse]

Summary

Fix intermittent false EMPTY_RESPONSE results from long Pro turns whose initial SSE stream completes before the final assistant message is available.

• Default fresh Pro text turns now use a saved conversation internally, poll it when the initial stream has no text, and delete it after a successful result.
• Explicit --temporary, --save, --keep-conversation, and continued-conversation behavior stays opt-in and unchanged.
• Recovery ignores hidden/tool/intermediate assistant messages and only reconciles fresh conversations, avoiding stale answers from continued chats.
• A polling failure reports the saved conversation ID and never resubmits the original quota-bearing prompt, even when --retries is enabled.

This builds on #1, extending reconciliation to the default call path where the reported failures occur while retaining clean ChatGPT history.

Verification

• bun run check
• 390 tests passed
• TypeScript tsc --noEmit passed

[cinnabarhorse]

Closing while the local empty-response recovery is validated against a real Pro turn. The branch is intentionally retained for reopening if validation passes.