Enable better options for subagent prompting #3574

Describe the feature or problem you'd like to solve

I want all codebase exploration to load a skill before doing any work. I could do this by injecting any sort of mandatory prompt into the explore agent's context, but my options are limited here. From empirical testing, subagents appear to ignore copilot-instructions.md and asking the main agent to pass along a prompt is unreliable. Another option is to create a custom agent and ask the main agent to use it instead of explore for exploration tasks, but this also does not work because the task tool can only spawn builtin agents.

Proposed solution

In order of preference:

• Allow me to specify custom instructions for built-in subagents, especially explore. (Note: the documentation implies that instructions apply to all agent contexts, but this does not appear to be true from empirical tests.)
• Allow the main agent to spawn a custom subagent with the task tool. Currently if it tries to do this the task tool fails.

Example prompts or workflows

N/A

Additional context

No response