Output GitHub Copilot custom instructions / skill files instead of raw prompts

[Alan-Jowett]

Idea

Currently PromptKit assembles a raw prompt (Markdown) that the user copies into a fresh LLM session. Instead, PromptKit could output GitHub Copilot custom instructions files (e.g. .github/copilot-instructions.md, skill definitions) along with appropriate format variants for other agent platforms.

Motivation

• Raw prompts are ephemeral — they live in a chat session and are lost when it ends.
• Copilot custom instructions / skill files are persistent, version-controlled, and automatically loaded by the agent runtime.
• This would make PromptKit a tool for authoring reusable agent behaviors, not just one-off prompts.

Possible scope

• Output .github/copilot-instructions.md\ for GitHub Copilot.
• Output platform-appropriate formats for other agent runtimes (Claude Code, Cursor, etc.).
• Template metadata could declare which output formats are supported.
• Bootstrap flow could ask the user which platform(s) they target.

Open questions

• Should this replace raw prompt output or be an additional output mode?
• Which agent platforms should be supported initially?
• How should platform-specific constraints (e.g., file size limits, supported syntax) be handled?