Disable appending builtin agent-types via configuration/options

### Describe the bug

Covers
- https://github.com/github/copilot-cli/issues/935
- https://github.com/github/copilot-cli/issues/1990
- https://github.com/github/copilot-cli/issues/1528

### Affected version

1.0.5, would believe this didnt cover in new version by looking changelog

### Steps to reproduce the behavior

See https://github.com/github/copilot-cli/issues/1528

### Expected behavior

For the issue above: it should be overriden;, My proposal is just add the option to disable builtin behavior.

### Additional context

Claude investigated this and results this cases as do require binary patching ultimately since it is not open source(while vscode copilot chat is). i hope this really investigated further.