Separation of concerns in agents #6156

(If an existing issue talks about this, you may close this issue, citing the issue)

An agent in VS Code-Github Copilot has tools and instructions. However, these come coupled with the agent. If I want to use different tools for different agents, I will have to create different agents. Similarly, if I have different instructions that is just slightly different (inheriting instructions), I can't do that without creating a separate agent. The default Plan agent is quite capable and I would like to extend it. No option other than copy-pasting the content of Plan agent and then adding my specific instructions in new agent.

This is the same problem web components have had. They package everything nicely, but if you want to reuse just one thing eg. style, you can't do so easily. Agents are the same form of packaging everything in one, so reusing becomes difficult.

Here's what I propose:

1. Named toolsets
2. Named instructions

The agent.md can be composed with named toolsets and named instructions. Of course, you can still have everything in one file. But as complexity grows, it is just better to refer to the instruction and toolsets files in the .agent.md file.