ci(deps): keep actions, docker base and pip deps updated automatically

Dependabot opens weekly PRs for the three update surfaces: github-actions
(grouped into one PR; the first one also clears the Node 20 deprecation
warning on checkout@v4 and upload-artifact@v4), the python base image, and
the pip dependencies. A companion workflow enables auto-merge for minor and
patch bumps so they land on their own once the required make test check is
green; major bumps stay manual.

Requires two repo settings: allow auto-merge, and a branch protection rule
on main with make test as required status check, which is the gate that
keeps auto-merge from landing untested changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,21 @@
+---
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      actions:
+        patterns:
+          - "*"
+
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: weekly
+
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: weekly