Add concurrency group to cancel outdated PR workflow runs

Cancels in-progress runs when a new push arrives on the same branch,
but leaves main runs to complete so coverage reports aren't dropped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: loks0n

.github/workflows/ci.yml

@@ -10,6 +10,10 @@ on:
     branches:
       - "*"
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
 jobs:
   build:
     if: "!contains(github.event.head_commit.message, 'skip ci')"