fix(detect-pipeline): use two-dot diff for shallow clone compatibility

Three-dot diff (A...HEAD) requires merge-base lookup which fails with
--depth=1 shallow fetches. Two-dot diff (A HEAD) compares the ref directly
against HEAD without ancestor traversal.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/detect-pr-pipeline.yaml

@@ -53,7 +53,7 @@ jobs:
           mkdir -p /tmp/pr-detect
           git fetch origin "$BASE_REF" --depth=1 2>/dev/null
           git fetch origin +refs/pull/*/head:refs/pull/*/head --depth=1 2>/dev/null
-          git diff --name-only "origin/$BASE_REF"...HEAD > /tmp/pr-detect/changed_files.txt
+          git diff --name-only "origin/$BASE_REF" HEAD > /tmp/pr-detect/changed_files.txt
           echo "Files found: $(wc -l < /tmp/pr-detect/changed_files.txt)"
           cat /tmp/pr-detect/changed_files.txt