Update autograder PR feedback flow

[danielrazavi]

Summary

This updates the assignment autograder feedback flow so passing submissions no longer require the GitHub Actions bot to create a formal PR approval.

Changes

• Always posts autograder results as a normal PR comment.
• Adds an explicit “approved by the autograder” message when all checks pass.
• Keeps REQUEST_CHANGES reviews for failing submissions.
• Adds workflow permissions needed for PR comments and request-changes reviews.
• Adds Python cache files/directories to .gitignore.

Why

GitHub blocks Actions-created approval reviews unless the repo setting “Allow GitHub Actions to create and approve pull requests” is enabled. Posting the passing result as a normal PR comment avoids requiring participants to change repository settings, while still preserving request-changes reviews for failed submissions.

[github-actions]

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