fix(opencode): Fix hardcoding default build OpenCode agent when sending responses

[cb-bradbeebe]

Summary

• Fixes [OpenCode] Allow customizing the review agent name that feedback gets sent back to instead of build #1123 by removing OpenCode's implicit fallback to build when agent switching is unset/disabled.
• Keeps execution on the current agent unless a valid explicit agent is configured.
• Prevents invalid/unavailable configured agents from being sent to OpenCode.

What changed

• Updated OpenCode agent-switch behavior so default/disabled mode does not set an agent parameter.
• Added validation for explicitly configured agents via client.app.agents() before request submission.
• If a configured agent is invalid/unavailable, the agent override is omitted and surfaced as a warning (log + toast).
• Updated related docs and tests.

Testing

• Unit tests
• Marketing docs builds
• Manual testing with local plugin build

Demo/Screenshots

Bug

Before

After

New warning toast if user manually enters a invalid custom agent via Settings

[backnotprop]

Thanks for the fix. We kept your validation path and the review-feedback behavior exactly as you built it: agents are validated through client.app.agents(), an unavailable agent is omitted with a log plus toast, and the empty custom-name case no longer leaks "custom" as an agent name.

One change on top: the agent switch cookie is shared by plan approval and code review, so flipping the stored default to disabled also removed the OpenCode plan-approval handoff for everyone who never touched the setting. I pushed a commit that makes the unset default surface-aware instead. Plan approval keeps defaulting to the build handoff, so existing plan-mode users see no change, and review feedback now defaults to staying on the current agent as you intended. An explicit choice in Settings still applies everywhere.