ci: add workflow_dispatch to notify-engine

Allow manually triggering the reverse submodule sync (dispatches with the
current main HEAD). This commit is also the first real-push end-to-end test
of the reverse direction now that ENGINE_TOKEN is configured.

Author: Seungpyo1007

.github/workflows/notify-engine.yml

@@ -17,6 +17,7 @@ on:
     branches: [main]
     paths-ignore:
       - "TechEngine"
+  workflow_dispatch:   # manual reverse-sync trigger (uses main HEAD sha)
 
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   contents: read