[External Mode] GitHub Action inputs, mode plumbing, and compatibility

[ArthurCRodrigues]

[External Mode] GitHub Action interface changes (inputs, env, validation, backwards compatibility)

Goal

Add a new Action execution mode (external / private) while preserving current repository-config mode.

Scope

• Update action.yml inputs:

  • execution-mode (repo default, external optional)
  • grading-config-id (required in external)
  • autograder-cloud-url (required in external)
  • autograder-cloud-token (required in external)

• Keep existing inputs for repo mode unchanged.

• Update github_action/entrypoint.sh:

  • Pass new arguments to Python entrypoint when present.
  • Validate mode-specific required env vars.

• Update github_action/main.py:

  • Parse new args.
  • Enforce mode-specific validation rules.
  • Route execution path based on mode.

Backwards Compatibility Requirements

• Existing GitHub Classroom workflows (repo mode) continue working without modifications.
• Existing required inputs keep same meaning in repo mode.

Acceptance Criteria

• repo mode remains default and stable.
• external mode fails fast with actionable errors when required params are missing.
• external mode receives and passes config ID/cloud URL/token to service layer.
• Unit tests cover arg validation and mode routing.