ci: collapse the push/pull_request double-fire in the concurrency group

`github.ref` cannot collapse a push and its pull_request run: the same commit
is refs/heads/<branch> on push and refs/pull/N/merge on the PR, so both go to
completion on identical bytes. `github.head_ref || github.ref` gives both events
one group key on a PR branch; head_ref is empty elsewhere, so pushes to the
default branch keep grouping by ref.

Marketing-wing half of the org-wide CI-minute sweep. Minutes here bill to the
personal account by design, so this is the one place the saving is not notional.
Per dogfood-swarm SKILL item 9.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: mcp-tool-shop

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 permissions: