Add CI workflow for updating regression test baselines

.github/workflows/update_regression_baselines.yaml

@@ -0,0 +1,44 @@
+name: Update Regression Baselines
+
+on:
+  workflow_dispatch:
+
+concurrency:
+  group: update-regression-baselines-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: write
+
+jobs:
+  update-baselines:
+    name: Update regression baselines
+    runs-on: ubuntu-24.04
+    timeout-minutes: 30
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
+        with:
+          ref: ${{ github.ref }}
+
+      - name: Set up Python
+        uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
+        with:
+          python-version: "3.14"
+          cache: pip
+
+      - name: Install testing dependencies
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e .[testing]
+
+      - name: Update regression baselines
+        run: pytest tests/regression --update-baselines
+
+      - name: Commit updated baselines
+        uses: stefanzweifel/git-auto-commit-action@778341af668090896ca464160c2def5d1d1a3eb0 # v6
+        with:
+          commit_message: "test(regression): update baselines"
+          commit_user_name: github-actions[bot]
+          commit_user_email: 41898282+github-actions[bot]@users.noreply.github.com

docs/developer_guide.rst

@@ -60,13 +60,46 @@ Run tests with coverage::
 
     pytest --cov CodeEntropy --cov-report=term-missing
 
-Update regression baselines::
+Run a specific test::
+
+    pytest tests/unit/.../test_file.py::test_function
+
+Updating Regression Baselines
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Regression baselines should only be updated when a change intentionally alters
+the expected numerical output of CodeEntropy. Do not update baselines simply
+because a regression test fails.
+
+Before updating baselines, confirm that:
+
+- The code change is intentional and understood.
+- The new output has been reviewed.
+- The regression difference is expected.
+- The pull request explains why the baselines changed.
+
+For local updates, run::
 
     pytest tests/regression --update-baselines
 
-Run a specific test::
+For pull requests, baselines can also be updated using the GitHub Actions
+workflow **Update Regression Baselines**.
 
-    pytest tests/unit/.../test_file.py::test_function
+To use it:
+
+1. Push your changes to your PR branch.
+2. Open the **Actions** tab on GitHub.
+3. Select **Update Regression Baselines**.
+4. Click **Run workflow**.
+5. Select your PR branch.
+6. Run the workflow.
+
+The workflow runs the regression tests with ``--update-baselines`` and commits
+any changed baseline files back to the selected branch.
+
+Only use this workflow when the baseline changes are intentional. If a regression
+test fails unexpectedly, investigate the failure instead of updating the
+baseline.
 
 Regression Test Data
 --------------------