fix set -euo pipefail failure

.github/workflows/build.yml

@@ -40,13 +40,18 @@ jobs:
             exit 0
           fi
 
-          # Determine changed files (PR vs push)
+          # Determine changed files (PR vs push vs manual dispatch)
           if [[ "${{ github.event_name }}" == "pull_request" ]]; then
             BASE_SHA="${{ github.event.pull_request.base.sha }}"
             HEAD_SHA="${{ github.event.pull_request.head.sha }}"
+          elif [[ "${{ github.event_name }}" == "push" ]]; then
+            # For push events try to get the previous commit; fall back to empty
+            BASE_SHA="$(git rev-parse HEAD~1 2>/dev/null || true)"
+            HEAD_SHA="$(git rev-parse HEAD 2>/dev/null || true)"
           else
-            BASE_SHA="$(git rev-parse HEAD~1 || echo '')"
-            HEAD_SHA="$(git rev-parse HEAD)"
+            # For workflow_dispatch and other events treat as "all files changed"
+            BASE_SHA=""
+            HEAD_SHA="$(git rev-parse HEAD 2>/dev/null || true)"
           fi
 
           if [[ -n "$BASE_SHA" ]]; then