Chat default fails instead of falling back to Terminal UI for unsupported harnesses

This was generated by AI during triage.

Summary

When Chat UI is configured as the default interface and a project is launched with a harness that Chat UI does not support, such as Kimi, AO attempts to start it in Chat and surfaces an error.

AO should detect that the selected harness is not supported by Chat UI and automatically launch the project in Terminal UI instead.

This appears to be a regression or an uncovered launch path related to #3712, which implemented proactive Terminal UI fallback for TUI-only agents in the new-task composer.

Steps to reproduce

1. Set the default interface to Chat UI.
2. Launch a project using a harness that Chat UI does not support, such as Kimi.
3. Observe that launch fails with an unsupported interface error.

Expected behavior

AO checks the selected harness capabilities before launch. If Chat UI is unsupported, it automatically starts the project in Terminal UI without first sending a guaranteed-to-fail Chat request.

Actual behavior

AO attempts to use the configured Chat UI default and displays an error instead of falling back to Terminal UI.

Acceptance criteria

• Harness capability is resolved before the launch request is sent.
• A harness unsupported by Chat UI launches directly in Terminal UI when Chat UI is the configured default.
• Chat-capable harnesses continue to respect the configured Chat UI default.
• The existing explicit interface behavior remains unchanged when the user has selected a supported interface.

Related

• #3712