feat(acp): expose subagents in available modes response

[YichenLi00]

Summary

Expose subagents in ACP available modes response for client-side access.

Closes #11576

Problem

ACP clients (e.g., agent-shell for Emacs) cannot access subagent information through ACP. Users need this to @mention specific subagents.

Solution

• Add optional mode field to ModeOption type
• Remove server-side subagent filter
• Clients can filter by mode field if they only want primary agents

Changes

- type ModeOption = { id: string; name: string; description?: string }
+ type ModeOption = { id: string; name: string; description?: string; mode?: string }

- .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

Existing clients continue to work. The mode field is optional and only adds information.

[github-actions]

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

No duplicate PRs found

[YichenLi00]

@rekram1-node Fixed the issues from #12305:

1. Added mode field to ModeOption type
2. Why expose subagents: ACP clients like Emacs agent-shell need this for @mentions. Server shouldn't decide visibility - clients can filter by mode field if needed.