Disable appending builtin agent-types via configuration/options

[ilsubyeega]

Describe the bug

Covers

• general purpose agent used instead of actual agent #935
• Built-in agents (explore, task, code-review) do not inherit custom instructions #1990
• Custom agents with builtin agent names are silently ignored #1528

Affected version

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

Steps to reproduce the behavior

See #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.

[ilsubyeega]

fwiw vsc extension have this: