Support Copilot "Prompt File" configurations

[yookoala]

Describe the feature or problem you'd like to solve

I want to make frequently used prompt reusable

Proposed solution

Support reading and creating custom slash command with Prompt files, as VSCode,

Example prompts or workflows

Much like VSCode, agent would accept /custom-prompt command if .github/prompts/custom-prompt.prompt.md exists in the working directory for the copilot CLI.

Additional context

No response