fix(cli): flush stream-json output

[hogeheer499-commits]

Related Issue

Resolves #1897

The stream-JSON writers in headless mode ignored the return value of stdout.write(). With a slow or piped consumer, the final assistant message or resume hint could still be buffered when signal cleanup called process.exit().

Both headless runners now wait for drain after a backpressured write. Prompt-mode shutdown also gives pending output a bounded chance to flush, and cleans up its listeners whether the wait succeeds or times out.

The new tests cover delayed drain, ordinary writes, listener cleanup, resume hints, and signal shutdown. I ran the full repository suite (2,326 passing, 5 skipped), typecheck, lint, changeset validation, and git diff --check.

This includes a patch changeset for @moonshot-ai/kimi-code. The stream-JSON interface itself has not changed, so no documentation update is needed.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 35beeb1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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