Add concurrency groups to CI and mark-stable workflows

Introduce explicit concurrency settings to the CI and mark-stable
workflows to serialize runs per repository and ref. This prevents
overlapping executions for the same branch or tag and makes pipeline
behavior more predictable during rapid pushes.

https://chatgpt.com/share/6988bef0-1a0c-800f-93df-7a6c1bdc0331

.github/workflows/ci.yml

@@ -6,6 +6,10 @@ on:
       - main
   pull_request:
 
+concurrency:
+  group: global-ci-${{ github.repository }}-${{ github.ref_name }}
+  cancel-in-progress: false
+
 jobs:
   test-unit:
     uses: ./.github/workflows/test-unit.yml

.github/workflows/mark-stable.yml

@@ -1,5 +1,9 @@
 name: Mark stable commit
 
+concurrency:
+  group: mark-${{ github.repository }}-${{ github.ref_name }}
+  cancel-in-progress: false
+
 on:
   push:
     branches: