Fix git checkout error by using commit hash instead of ref

The workflow was failing with "pathspec 'refs/pull/170/merge' did not match any file(s)" because:
- GitHub Actions passes ${{ github.ref }} which is 'refs/pull/170/merge' for PRs
- This is a GitHub-specific reference that exists remotely but can't be checked out as a local branch
- After pushing to pr-screenshot branch, script tried to checkout this ref and failed

Fixed by:
- Saving the current commit hash (git rev-parse HEAD) before switching branches
- Using that commit hash to return to the original state (works in detached HEAD mode)
- This approach works regardless of whether we're on a branch, tag, or PR merge ref

The workflow will now:
1. Save current commit hash
2. Switch to pr-screenshot branch and push screenshots
3. Return to the saved commit hash (detached HEAD is fine in CI)
4. Continue with subsequent workflow steps that check for changes and comment on PR

Co-authored-by: richardthe3rd <573334+richardthe3rd@users.noreply.github.com>

Author: Copilot

scripts/update_screenshot_branch.sh

@@ -26,6 +26,9 @@ echo "Source branch: $SOURCE_BRANCH"
 # Store the current directory to return to it
 ORIGINAL_DIR=$(pwd)
 
+# Store the current commit hash to return to it later
+ORIGINAL_COMMIT=$(git rev-parse HEAD)
+
 # Copy screenshots to a temp directory BEFORE switching branches
 TEMP_DIR=$(mktemp -d)
 echo "Copying screenshots to temporary directory: $TEMP_DIR"
@@ -109,6 +112,7 @@ else
   echo "✓ Screenshots updated successfully to $BRANCH_NAME/$PR_FOLDER"
 fi
 
-# Return to the original branch
-echo "Returning to source branch: $SOURCE_BRANCH..."
-git checkout "$SOURCE_BRANCH"
+# Return to the original commit
+# Use the commit hash we saved earlier, which works regardless of branch/ref type
+echo "Returning to original commit: $ORIGINAL_COMMIT..."
+git checkout "$ORIGINAL_COMMIT"