Add fine-grained system prompt customization (customize mode)#816

MackinnonBuck wants to merge 4 commits intoupdate-copilot-1.0.7github/copilot-sdk:update-copilot-1.0.7from

mackinnonbuck/prompt-customizationgithub/copilot-sdk:mackinnonbuck/prompt-customizationCopy head branch name to clipboard