fix(opencode): flush stdio before cli exit

[YOMXXX]

Issue for this PR

Closes #29330
Closes #26399

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The CLI explicitly calls process.exit() after command handling to avoid hanging subprocesses. Large stdout writes can still be buffered at that point when output is piped, so commands such as opencode export <id> | jq or opencode debug skill | jq can see truncated JSON.

This PR adds a small stdio flush helper and waits for stdout/stderr write callbacks before the final forced exit. The explicit exit behavior is preserved, but pending buffered CLI output gets a chance to drain first.

How did you verify your code works?

• cd packages/opencode && PATH="$HOME/.bun/bin:$PATH" bun test test/cli/stdout.test.ts
• cd packages/opencode && PATH="$HOME/.bun/bin:$PATH" bun typecheck
• git diff --check
• PATH="$HOME/.bun/bin:$PATH" .husky/pre-push

Screenshots / recordings

Not applicable; CLI stdout flushing only.

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:

Related PR Found:

• PR #26389: fix: flush debug skill stdout
  • This appears to be related as it addresses flushing stdout for the debug skill command, which is one of the commands mentioned in the current PR (fix(opencode): flush stdio before cli exit #29340) that can experience truncated output when piped to tools like jq.

The current PR (29340) is a broader fix that addresses the root cause for multiple commands by implementing a general stdio flushing mechanism before CLI exit, whereas PR #26389 may be a more targeted fix for the debug skill command specifically.