Add concurrency control to prevent redundant workflow runs

Implement workflow-level concurrency control to optimize CI resource usage:

- Group by workflow name and git ref (branch/tag)
- Cancel in-progress runs when new commits are pushed
- Prevents multiple CI runs for the same branch running simultaneously

This addresses the issue where rapid pushes to the same branch would
spawn multiple workflow instances, wasting GitHub Actions runner time
and delaying feedback on the latest commit.

Example behavior:
- Push A triggers workflow run 1
- Push B to same branch triggers workflow run 2
- Run 1 is automatically cancelled, run 2 proceeds

Concurrency group format: "CI-refs/heads/branch-name"

Author: jserv

.github/workflows/main.yml

@@ -2,6 +2,10 @@ name: CI
 
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
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