ci: migrate tests to reusable workflows and introduce stable-tag pipeline

- convert all test workflows to reusable workflow_call
- add central CI workflow for branches and PRs
- add mark-stable workflow triggered on main pushes
- ensure stable tag updates only after all tests succeed
- remove duplicated triggers from test workflows
`

https://chatgpt.com/share/693aa4a6-7460-800f-ba47-cfc15b1b2236

.github/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+
+on:
+  push:
+    branches-ignore:
+      - main
+  pull_request:
+
+jobs:
+  test-unit:
+    uses: ./.github/workflows/test-unit.yml
+
+  test-integration:
+    uses: ./.github/workflows/test-integration.yml
+
+  test-container:
+    uses: ./.github/workflows/test-container.yml
+
+  test-e2e:
+    uses: ./.github/workflows/test-e2e.yml
+
+  test-virgin-user:
+    uses: ./.github/workflows/test-virgin-user.yml
+
+  test-virgin-root:
+    uses: ./.github/workflows/test-virgin-root.yml