fix(cli): drain stdout in export to prevent truncation (fixes #29330)

[levgiorg]

Issue for this PR

Closes #29330

Type of change

• Bug fix

What does this PR do?

Fix opencode export producing truncated JSON output when piped. Ensures stdout is fully drained before the process exits so large session exports are not cut off at pipe boundaries.

How did you verify your code works?

• All CI checks pass (check-duplicates, check-standards, add-contributor-label, check-compliance)
• TypeScript typecheck passes
• Existing tests pass

Screenshots / recordings

Not applicable — no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate found:

• PR fix(opencode): flush stdio before cli exit #29340: fix(opencode): flush stdio before cli exit
  • fix(opencode): flush stdio before cli exit #29340
  • This appears to be related—both PRs address stdout/stdio flushing issues that prevent truncation of output during CLI exit. They're likely addressing the same or very similar problem from different angles.

[github-actions]

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