ci(preview-readme): skip fork PRs to fix checkout failure

The preview-readme job checks out the head branch by name
(ref: github.event.pull_request.head.ref) without setting repository, so
actions/checkout fetches refs/heads/<branch> from this repo. For a fork PR
the branch lives only in the fork, so the fetch finds no ref and the step
fails with 'git failed with exit code 1' (seen on PR #28, from
gaul/s3proxy-chart). Every other workflow uses the default merge-ref
checkout and is unaffected.

Even with checkout fixed, the comment-posting steps need a write-scoped
GITHUB_TOKEN, which the pull_request event does not grant fork PRs, so the
preview feature is inherently limited to same-repo branches.

Guard the job on head.repo.full_name == github.repository: internal
branches still get the README preview; fork PRs skip it cleanly instead of
reporting a spurious failure.

Author: CRThaze

.github/workflows/preview-readme.yaml

@@ -10,6 +10,14 @@ on:
 
 jobs:
   preview-readme:
+    # This job checks out the head branch by name and posts the rendered README
+    # diff back as a PR comment. Both of those require the head branch to live in
+    # this repo and a write-scoped GITHUB_TOKEN. Neither holds for fork PRs: the
+    # branch exists only in the fork (so `actions/checkout` with a bare `ref:`
+    # fetches refs/heads/<branch> from this repo and fails), and the
+    # pull_request event grants forks a read-only token (so createComment 403s).
+    # Skip forks; internal branches still get the preview.
+    if: github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps: