fix(ci): add concurrency group to cancel superseded runs on force-push (#444)

Force-pushes triggered a new pull_request synchronize event while the
previous run's lightweight jobs (conventional-commits, IPR) still held
the free-tier runner slots. The 4-runner Test Python matrix for the new
SHA was queued but never scheduled — the fast jobs always won. Required
a manual close/reopen to retrigger.

The concurrency block cancels in-progress runs for the same ref so the
new SHA's matrix jobs get scheduled immediately.

Closes #413

Author: bokelley

.github/workflows/ci.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [main]
 
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test:
     name: Test Python ${{ matrix.python-version }}