ci: add concurrency cancel-in-progress to ci.yml

Cancels any in-progress CI run for the same branch when a new commit
arrives, cutting queue time from O(n × job_duration) to ~0.

Closes #497

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

.forgejo/workflows/ci.yml

@@ -4,6 +4,9 @@ on:
     branches:
       - main
   pull_request:
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   check:
     name: Full Project Check