[lint apply] Add PR-only pre-commit workflow

Add .github/workflows/format.yml: a GitHub Actions workflow that runs pre-commit only for changed files in PRs. It triggers on pull_request (opened, synchronize, reopened), uses a detect_changes job to checkout full history and compute the list of files changed against the base branch, and exposes that list as an output. A precommit job (guarded by a condition that changed files exist) checks out the PR branch, sets up Python 3.12, installs pre-commit, and runs pre-commit only on the changed files.

Author: C-Achard

.github/workflows/format.yml

@@ -0,0 +1,55 @@
+name: pre-commit (PR only on changed files)
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+jobs:
+
+  detect_changes:
+    runs-on: ubuntu-latest
+
+    outputs:
+      changed: ${{ steps.changed_files.outputs.changed }}
+
+    steps:
+      - name: Checkout full history
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Detect changed files
+        id: changed_files
+        run: |
+          git fetch origin ${{ github.base_ref }}
+          CHANGED_FILES=$(git diff --name-only origin/${{ github.base_ref }}...HEAD)
+          echo "changed=$CHANGED_FILES" >> $GITHUB_OUTPUT
+
+      - name: Show changed files
+        run: |
+          echo "Changed files: ${{ steps.changed_files.outputs.changed }}"
+
+
+  precommit:
+    runs-on: ubuntu-latest
+    needs: detect_changes
+
+    if: ${{ needs.detect_changes.outputs.changed != '' }}
+
+    steps:
+      - name: Checkout PR branch
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+          ref: ${{ github.head_ref }}
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.12"
+
+      - name: Install pre-commit
+        run: pip install pre-commit
+
+      - name: Run pre-commit on changed files
+        run: pre-commit run --files ${{ needs.detect_changes.outputs.changed }}