fix: default to linux-only QA, full 3-OS only via qa-full label

Reduces per-run cost from ~$10-16 to ~$2.50 by defaulting to
Linux-only. Use qa-full label or workflow_dispatch for 3-OS runs.

.github/workflows/pr-qa.yaml

@@ -47,11 +47,6 @@ jobs:
             echo "skip=true" >> "$GITHUB_OUTPUT"
           fi
 
-          # TODO: remove push trigger before merge
-          if [ "$EVENT_NAME" = "push" ]; then
-            FULL=true
-          fi
-
           # Full QA triggers
           if [ "$EVENT_NAME" = "workflow_dispatch" ] && \
              [ "$INPUT_MODE" = "full" ]; then