Remove invalid expression default from GITHUB_TOKEN input

[tarmojussila]

Expressions like ${{ github.token }} are not evaluated in action.yml inputs. The token is already available via process.env.GITHUB_TOKEN in the action code as a fallback.

[github-actions]

Z.ai Code Review

Review Feedback

1. Potential Breaking Change & DX Regression
Removing the default value for GITHUB_TOKEN creates a breaking change for any workflows currently using this action without explicitly passing the token.

• Impact: Users will now receive an empty string/undefined for this input, likely causing API calls to fail (401 Unauthorized).
• Recommendation: Unless there is a strict security reason to force explicit manual configuration, keep default: ${{ github.token }}. It significantly improves developer experience by reducing boilerplate setup.

2. Missing Input Validation
Since the input is now required: false and has no default, the action code must explicitly handle cases where the token is missing.

• Risk: The code likely attempts to authenticate with the GitHub API immediately. If the input is an empty string, the application may throw an unhandled exception or cryptic error.
• Action: Ensure dist/index.js checks if GITHUB_TOKEN is populated before use and provides a clear error message (e.g., "GITHUB_TOKEN is required for this action to run") if it is missing.