AI: Make it possible to exclude default rule

[taoeffect]

Summary

The agent panel automatically uses the default rule when a command is given. This is frequently undesirable.

Description

Steps to trigger the problem:

1. Use the agent panel to do something.

Actual Behavior:

It uses the default rule and there's no way to disable this.

Expected Behavior:

Please see the suggestion in discussion #30187. In plaintext threads we can edit the full contents of the query before it's submitted to the LLM. Ideally we'd use the plaintext approach to interact with the agent as well, as it avoids many problems and is more powerful, while at the same time not needing much custom UI.

Zed Version and System Specs

Zed: v0.185.15 (Zed) OS: macOS 14.7.3 Architecture: aarch64