ci: re-enable QA workflow on phase-6 push, add workflow_dispatch

Author: Sam-Bolling

.github/workflows/qa.yml

@@ -3,7 +3,8 @@ on:
   push:
     branches:
       - main
-      # - phase-6  # tested and passing as of 2026-02-24; disabled to avoid triggering on every push
+      - phase-6
+  workflow_dispatch:
   pull_request:
     types: [opened, synchronize, ready_for_review]