fix(opencode): CLI .fail() handler swallows error message

[zevision]

Description

When passing an invalid argument like --unkown-flag, opencode shows the full help text but never tells the user what they did wrong. The error message msg is passed into the .fail() handler but never written to stderr.
Before the fix, opencode --unkown-flag and opencode --help produce identical output — the user has no way to know their argument was wrong.
The issue is in packages/opencode/src/index.ts:181-192. The .fail() handler calls cli.showHelp(show) but never outputs msg.

Plugins

No response

OpenCode version

1.15.10

No response

Steps to reproduce

1. Run opencode --unkown-flag
2. Observe there is no error message telling you the argument is unknown
3. Compare with opencode --help — the output is identical

Screenshot and/or share link

No response

Operating System

Windows 10

Terminal

cmd