Add support for GitHub Models prompt files

[FidelusAleksander]

Add support for GitHub Models .prompt.yml files

I'm imagining the interface could be like:

steps:
  - name: Prompt
    id: prompt
    uses: FidelusAleksander/prompt-action@v1
    with:
      prompt-file: './.github/prompts/sample.prompt.yml'
      input: |
         var1: hello
         var2: ${{ steps.<some-step>.outputs.<output> }}
         var3: |
           Lorem Ipsum
           Hello World

[github-actions]

👋 Hello there!

Thank you, @FidelusAleksander, for opening this issue and sharing your idea!

This appears to be a feature request for adding support for GitHub Models .prompt.yml files. I appreciate the detailed interface example—it helps bring your suggestion to life. To better understand its potential impact, could you elaborate on the specific use cases or scenarios where this feature would be particularly beneficial?

Looking forward to your input!

Best regards,
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