Add merged-PR closed trigger for site publish evaluation (#59)

## Summary
- add pull_request.closed to .github/workflows/Process-PSModule.yml
- keep existing push and PR update triggers unchanged

## Why
Process-PSModule computes Publish.Site.Enabled from merged PR context.
Including the closed PR event enables the workflow to evaluate and
execute publish behavior on merge events.

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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug

.github/workflows/Process-PSModule.yml

@@ -11,6 +11,7 @@ on:
     branches:
       - main
     types:
+      - closed
       - opened
       - reopened
       - synchronize