[FEATURE] Ability to disable default sub-agents

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I am having terrible experience with the plan subagent. Like very very very frustrating. Would really appreciate if i could just disable it.

Proposed Solution

Just make default agents be able to be disabled, like we can do with custom ones.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

_No response_

Additional Context

_No response_