ci: key concurrency groups on PR number or ref, not head_ref

github.head_ref is empty on tag pushes and manual runs (every run then
shares one group and cancels the last) and collides across forks when
two PRs share a branch name. pull_request.number is unique per PR;
github.ref covers push/tag/dispatch. Applied to all workflows using the
pattern, including pre-existing ones.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/workflows/vacuum-integration-tests.yml

@@ -19,7 +19,7 @@ on:
       - '.github/workflows/vacuum-integration-tests.yml'
 
 concurrency:
-  group: ${{ github.head_ref }}/vacuum-integration-tests
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 
 permissions: