fix(opencode): fail closed on unknown --agent

[kvnloo]

Issue for this PR

Closes #47038

Type of change

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

What does this PR do?

"opencode run --agent NAME" currently falls back to the default agent when the requested agent is unknown or is a subagent.

Fail closed instead. Exact and case-insensitive primary-agent names still resolve normally.

Based on #47091 by @RaviTharuma.

How did you verify your code works?

"cd packages/opencode && bun test test/cli/run/agent-flag.test.ts"

Tests cover exact matching, case-insensitive matching, unknown agents, and subagent rejection.

Screenshots / recordings

N/A

Checklist

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

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 72 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Hey! Your PR title Fix/run agent fail closed v3 doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

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

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 72-hour window.

Feel free to open a new pull request that follows our guidelines.