Add example showing how to pass env vars and secrets to the workflow step

[DeDuckProject]

Problem

GitHub Actions has two distinct variable types with different syntax:

• Secrets (sensitive): ${{ secrets.MY_SECRET }}
• Repository variables (non-sensitive): ${{ vars.MY_VAR }}

The README doesn't show a concrete example of passing either to the git-glimpse step via env:. Users who store their preview URL as a repository variable (a natural choice since it's not sensitive) will use secrets.* by habit and get an empty string silently — which then causes a confusing downstream error.

Suggested fix

Add a snippet to the README showing both cases:

- uses: DeDuckProject/git-glimpse/packages/action@main
  env:
    ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
    # Repository variable (Settings → Variables → Actions):
    PREVIEW_URL: ${{ vars.PREVIEW_URL }}
    # Secret (Settings → Secrets → Actions):
    AUTH_TOKEN: ${{ secrets.AUTH_TOKEN }}

A short note explaining the difference (vars.* vs secrets.*) would save users from a silent failure that's hard to trace.