I came across a nice little solution in copilot X and copilot 365/pro for including and referencing custom instructions to add behavior/formatting support to copilot (e.g. edit mode, article writing mode, documentation writting mode etc.)
I feel like the explicitly go out of their away to avoid building in custom instructions/increased prompt context to lower their costs so this is a bit of a work around.
copilot scans open files so I mention the file name in my prompt or reference the file explicitly using the reference file option in the chat window in the intellij copilot chat beta.
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Follow the instructions for preparing responses from the COPILOT.md file in how you answer and format your answer to the following prompt.