chore: integrate filtermail's .github directory into relay's

.github/workflows/chatmaild-ci.yml

@@ -0,0 +1,35 @@
+name: chatmaild CI
+
+on:
+  # Triggers when a PR is merged into main or a direct push occurs
+  push:
+    branches: [ "main" ]
+    
+  # Triggers for any PR (and its subsequent commits) targeting the main branch
+  pull_request:
+    branches: [ "main" ]
+
+permissions: {}
+
+# Newest push wins: Prevents multiple runs from clashing and wasting runner efforts
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  tox:
+    name: isolated chatmaild tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        # Checkout pull request HEAD commit instead of merge commit
+        # Otherwise `test_deployed_state` will be unhappy.
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          persist-credentials: false
+      - name: download filtermail
+        run: curl -L https://github.com/chatmail/filtermail/releases/download/v0.7.4/filtermail-x86_64 -o /usr/local/bin/filtermail && chmod +x /usr/local/bin/filtermail
+      - name: run chatmaild tests 
+        working-directory: chatmaild
+        run: pipx run tox
+