fix(opencode): print CLI parser errors

[YOMXXX]

Issue for this PR

Closes #29390

Type of change

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

What does this PR do?

Prints the yargs parser error message before showing top-level help for invalid CLI arguments.

Before this change, opencode --unkown-flag exited non-zero but produced the same visible output as opencode --help, so users could not tell what was wrong. The fail handler already receives the parser message; this writes it to stderr before rendering help.

How did you verify your code works?

• PATH="$HOME/.bun/bin:$PATH" bun test test/cli/invalid-args.test.ts
• PATH="$HOME/.bun/bin:$PATH" bun test test/cli/invalid-args.test.ts test/cli/help/help-snapshots.test.ts test/cli/smokes/read-only.test.ts
• PATH="$HOME/.bun/bin:$PATH" bun typecheck
• git diff --check

I also ran .husky/pre-push; it failed in the unrelated @opencode-ai/console-support package because local dependencies/types such as @solidjs/meta, @solidjs/router, solid-js, vite, and generated @opencode-ai/console-core/... modules were missing. The opencode package checks above passed.

Screenshots / recordings

N/A - CLI stderr behavior 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:

Potential duplicate PRs found:

1. fix(cli): output error message on .fail() (fixes #29390) #29400 - fix(cli): output error message on .fail() (fixes #29390)

   • fix(cli): output error message on .fail() (fixes #29390) #29400
   • Addresses the same issue (fix(opencode): CLI .fail() handler swallows error message #29390) with a similar fix scope

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

   • fix(opencode): show error message in CLI .fail() handler #29393
   • Also targets the same problem: showing error messages in the CLI fail handler

Why they're related: All three PRs (#29418, #29400, #29393) address the same issue (#29390) of displaying CLI parser error messages. PR #29418 (current) and the other two are likely duplicate efforts to fix the invalid CLI arguments error display problem. You may want to check if one of these has already been merged or if they should be consolidated.

[YOMXXX]

Checked the duplicate candidates.

• fix(cli): output error message on .fail() (fixes #29390) #29400 is already closed.
• fix(opencode): show error message in CLI .fail() handler #29393 targets the same CLI fail path, but this PR adds regression coverage through the real CLI subprocess path in test/cli/invalid-args.test.ts. The test verifies the original user-visible failure mode: unknown top-level args exit non-zero, keep stdout empty, and print both the parser error and help to stderr.

So the implementation scope is intentionally small, but the difference is that this PR keeps the behavior covered at the CLI boundary that regressed.