fix(cli): output error message on .fail() (fixes #29390)

[levgiorg]

When passing invalid arguments like --unknown-flag, the .fail() handler received the error message but never wrote it to stderr, making it impossible for the user to know what went wrong. This writes the error message to process.stderr before showing help or exiting.

Fixes #29390

[github-actions]

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[github-actions]

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

Potential Duplicate Found

PR #29393: fix(opencode): show error message in CLI .fail() handler
#29393

Why it's related: This PR appears to address the same issue as the current PR (#29400). Both are fixing the CLI .fail() handler to output error messages, which matches the exact problem described in your PR (not showing error messages on invalid flags like --unknown-flag). This looks like a direct duplicate addressing issue #29390.

[github-actions]

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