fix: remove secrets context expression from composite action input description

[Copilot]

GitHub Actions evaluates ${{ }} expressions in action metadata at load time, but the secrets context is not available in composite action input descriptions — causing the runner to fail before any steps execute.

Change

Removed the expression delimiters from the github-token input description in action.yml:

# Before
description: >
  GitHub token used to create the pull request.
  Pass ${{ secrets.GITHUB_TOKEN }} or a PAT from your workflow.

# After
description: >
  GitHub token used to create the pull request.
  Pass secrets.GITHUB_TOKEN or a PAT from your workflow.

This makes the description plain text, which is its only valid use — descriptions are documentation, not evaluated expressions.