Add Mergify merge queue configuration

.mergify.yml

@@ -0,0 +1,22 @@
+# Nothing is queued automatically: a maintainer ticks the box on Mergify's
+# comment (or comments `@mergifyio queue`) and the queue lands it once the
+# checks below are green.
+queue_rules:
+  - name: default
+    queue_conditions:
+      - base = main
+      - -draft
+    # Re-checked at the front of the queue, so a green run from before a rebase
+    # does not count.
+    merge_conditions:
+      - check-success = check
+    # Linear history
+    merge_method: rebase
+    update_method: rebase
+    batch_size: 1
+
+# Already the default for `queue`: only maintainers can add to the queue.
+commands_restrictions:
+  queue:
+    conditions:
+      - sender-permission >= write