Add concurrency control to serialize CI runs on main branch

[Copilot]

Ensures only one CI pipeline runs at a time on main branch to prevent concurrent deployments. Other branches (PRs) continue to run in parallel.

Changes

• Added concurrency group to ci.yml workflow
  • main branch: all runs share group ci-main, enforcing serialization
  • Other branches: unique group per ref via format('ci-{0}', github.ref), allowing parallelism
  • cancel-in-progress: true cancels stale runs when new commits arrive

concurrency:
  group: ${{ github.ref == 'refs/heads/main' && 'ci-main' || format('ci-{0}', github.ref) }}
  cancel-in-progress: true

Original prompt

Can you update the CI to ensure that there is a single pipeline running at the time when on main. Other branches can run in parallel.

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