Git operations inside the Docker container have no auth credentials
(actions/checkout@v5 stores them in $RUNNER_TEMP, not mounted).
Instead of fighting git auth, fetch the diff directly from the
Gitea API: GET /repos/{owner}/{repo}/pulls/{index}.diff
This uses the same token already passed for posting comments.
No pre-fetch workflow step needed. No git required in the container.
Also filters excluded patterns (lockfiles, etc.) from the API diff.
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