chore(ci): stop merged PRs' cleanup runs from cancelling main

[sam-goodwin]

Main's CI for #6 (and #5) was cancelled after 3s:

Canceling since a higher priority waiting request for ci-refs/heads/main exists

A merged PR's closed event (the cleanup run) has github.ref == refs/heads/main, so it landed in main's concurrency group with cancel-in-progress: true and cancelled the push run.

 concurrency:
-  group: ci-${{ github.ref }}
+  # Grouped by PR number, not github.ref: a merged PR's `closed` run has
+  # github.ref = refs/heads/main and would otherwise cancel main's push run.
+  group: ci-${{ github.event.pull_request.number && format('pr-{0}', github.event.pull_request.number) || github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}

Merging this runs main's CI, which deploys #6 to prod and re-records the README demo.

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