fix(cli): deflake prompt and background process races

[markijbema]

What

Deflake two macOS-sensitive CLI test paths:

• Isolate the prompt cancellation race fixture from real agent and skill discovery.
• Make persistent background process readiness output durable before forwarding it to the parent process.

Why

The prompt test verifies assistant finalization when cancellation interrupts processor creation, but it used the full agent layer before reaching the blocking processor. On slower CI hosts, unrelated agent initialization could consume the per-test budget before the actual race was reached.

Persistent background process readiness could also resolve from the runner's forwarded stdout before the same bytes were flushed to the persistent log file. Re-adoption then saw an empty log-backed output even though readiness had already been reported.

Notes

The prompt fixture still exercises the real prompt loop, run state, runner cancellation, and message persistence paths. The background process change preserves ordering by writing to the persistent log before forwarding output to the parent.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• packages/opencode/src/kilocode/background-process/runner.ts
• packages/opencode/test/session/prompt.test.ts

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_{Reviewed by claude-sonnet-5-20260630 · Input: 6.8K · Output: 13.4K · Cached: 1.6M}

_{Review guidance: REVIEW.md from base branch main}