EditWithInstructions allow prompt injection

Hello, very good plugin. Extra kudos for the effort you put into the nicer UX.

I think It will be nice if we can configure some 'prompt injection' when you use this feature.
Many times I want to add more code using the current one as context, but rather than including everything, the newly generated code most of the times only includes the new stuff. I have to instruct the bot everytime with something like `make sure to also include the existing parts of the code that are already correct` or stuff like that.
Not sure if prompt-injection is the best workaround, but it's the only thing I can think of

Start by locating the EditWithInstructions entry point and reading how its prompt and configuration are assembled. Determine where a user-configurable instruction could be applied, then verify that the configured text is included when the feature generates an edit.

Written by the indexing model from the issue text.