feat(acp): include subagents in available modes response

[YichenLi00]

Summary

• Include subagents in ACP available modes response

Closes #11576

Changes

Remove subagent filter from loadAvailableModes and add mode field to response, letting clients decide what to display.

- .filter((agent) => agent.mode !== "subagent" && !agent.hidden)
+ .filter((agent) => !agent.hidden)
  .map((agent) => ({
    id: agent.name,
    name: agent.name,
    description: agent.description,
+   mode: agent.mode,
  }))

Backward Compatibility

Same data shape, just with more entries and an additional mode field. Existing clients can filter by mode if needed.

[github-actions]

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

No duplicate PRs found

[rekram1-node]

ModeOption doesnt even have a mode field

also I dont understand, why would a subagent be in the selectable list. That seems inconsistent