Fallback when async prompt admission fails

[crankshift]

Summary

• Fall back to synchronous session.prompt when OpenCode rejects async prompt admission for /prompt_async.
• Keep completion timeout and assistant-message failures as errors after async admission succeeds.
• Add a regression test and bump release metadata to 0.11.2.

Test Plan

• pnpm run lint
• pnpm test
• pnpm run check

Closes #55