fix(opencode): show error message in CLI .fail() handler

[zevision]

Issue for this PR

Closes #29390

Type of change

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

What does this PR do?

The CLI .fail() handler receives the error message in msg but never writes it to stderr before showing help. Added UI.error(msg) before cli.showHelp(show) so users see what argument was wrong.

How did you verify your code works?

Ran bun run ./src/index.ts --unknown-flag and confirmed the output now shows Error: Unknown arguments: unknown-flag before the help text.

Screenshots / recordings

N/A — CLI change only

Checklist

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

[zevision]

Hi, I opened #29393 for this fix with the correct PR template (including issue ref, description, testing steps, checklist). I noticed the same fix was later committed directly to dev in 90a8c07 without going through my PR. Could you help me understand why? Happy to improve next time. Thanks.

[github-actions]

Automated PR Cleanup

Thank you for contributing to opencode.

Due to the high volume of PRs from users and AI agents, we periodically close older PRs using automated criteria so maintainers can focus review time on the most active and community-supported contributions.

This PR was closed because it matched the following cleanup criteria:

• The PR was created more than 1 month ago
• The PR had fewer than 2 positive reactions
• Positive reactions are counted as thumbs-up, heart, celebration, or rocket reactions on the PR

PRs created within the last month are not affected by this cleanup.

If you believe this PR was closed incorrectly, or if you are still actively working on it, please leave a comment explaining why it should be reopened. A maintainer can review and reopen it if appropriate.

Thanks again for taking the time to contribute.