Customized system prompt

[NuanceLab]

Thanks for the excellent extension. Would you consider making the system prompt editable in the settings?

Here are two use cases:

1. Smaller models often need guidance to properly recognize _CURSOR_HERE_ rather than rewriting entire code blocks; a custom prompt could resolve this.
2. Some users prefer minimal inline comments and docstrings, which are currently restricted by the default system prompt.

Adding this customization would be a significant help for users looking to tailor the extension to their specific workflow and needs.