ci: add dependabot, conventional commits check, and scheduled weekly tests

- dependabot.yml: weekly auto-PRs for pip and github-actions deps
  (major version bumps ignored, reviewed manually)
- conventional-commits.yml: blocks PRs with non-conforming titles;
  warns on individual commits (required for release-please changelogs)
- scheduled-tests.yml: weekly pytest run on py3.10+3.11, plus
  dependency vulnerability audit via safety

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,26 @@
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "06:00"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+    ignore:
+      # Ignore major version bumps — review manually
+      - dependency-name: "*"
+        update-types: ["version-update:semver-major"]
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "06:00"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+      - "github-actions"