ci: Reintroduce ci.yml workflow as the "must-be-green" CI entry point

Author: thomass-dev

.github/workflows/backend.yml

@@ -1,22 +1,6 @@
 name: backend
 
-on:
-  pull_request:
-    paths:
-      - '.github/workflows/backend.yml'
-      - 'skore/**'
-  push:
-    branches:
-      - main
-  merge_group:
-    types: [checks_requested]
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
-
-permissions:
-  contents: read
+on: [workflow_call]
 
 defaults:
   run:

.github/workflows/ci.yml

@@ -0,0 +1,56 @@
+name: ci
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  merge_group:
+    types: [checks_requested]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions: {}
+
+jobs:
+  changes:
+    runs-on: ubuntu-latest
+    outputs:
+      backend: ${{ steps.filter.outputs.backend }}
+    permissions:
+      pull-requests: read
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - uses: dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36 # v3.0.2
+        id: filter
+        with:
+          filters: |
+            backend:
+              - '.github/workflows/backend.yml'
+              - 'skore/**'
+
+  lint:
+    uses: ./.github/workflows/lint.yml
+    permissions:
+      contents: read
+
+  backend:
+    needs: [lint, changes]
+    if: ${{ (github.event_name == 'push' && github.ref == 'refs/heads/main') || (contains(fromJSON('["pull_request", "merge_group"]'), github.event_name) && needs.changes.outputs.backend == 'true') }}
+    uses: ./.github/workflows/backend.yml
+    permissions:
+      contents: read
+
+  ci-all-green:
+    needs:
+      - changes
+      - lint
+      - backend
+    if: ${{ always() }}
+    runs-on: Ubuntu-latest
+    steps:
+      - shell: bash
+        run: |
+          [[  ${{ contains(needs.*.result, 'failure') }} = false ]]

.github/workflows/lint.yml

@@ -1,19 +1,6 @@
 name: lint
 
-on:
-  pull_request:
-  push:
-    branches:
-      - main
-  merge_group:
-    types: [checks_requested]
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
-
-permissions:
-  contents: read
+on: [workflow_call]
 
 defaults:
   run:

.github/workflows/pr-lint-title.yml

@@ -8,7 +8,7 @@ permissions:
   pull-requests: read
 
 jobs:
-  lint-title:
+  pr-lint-title:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/setup-node@v4