chore(ci): dependabot config and auto-merge for routine bumps

.github/dependabot.yml

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+# Dependency updates.
+#
+# Grouped on purpose: one pull request per ecosystem per week, rather than one
+# per package. A single package's CVE list can otherwise open dozens of pull
+# requests in a morning. Security updates still arrive immediately, outside
+# this schedule, because Dependabot handles those separately.
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    groups:
+      routine:
+        update-types: ["patch", "minor"]
+    commit-message:
+      prefix: "chore(deps)"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      routine:
+        update-types: ["patch", "minor", "major"]
+    commit-message:
+      prefix: "ci(deps)"

.github/workflows/dependabot-auto.yml

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+# Routine dependency pull requests merge on their own once CI passes.
+#
+# Scope, stated plainly: a pull request opened by Dependabot whose update type
+# is patch or minor merges without a human reading it, after every required
+# check has passed. CI is the evidence for those merges. Major bumps get a
+# comment and wait for a maintainer.
+name: dependabot-auto
+
+on: pull_request_target
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto:
+    if: github.actor == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: read the update type
+        id: meta
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: enable auto-merge for patch and minor
+        if: contains(fromJSON('["version-update:semver-patch","version-update:semver-minor"]'), steps.meta.outputs.update-type)
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr merge --auto --squash "${{ github.event.pull_request.html_url }}"
+
+      - name: leave a major bump for a maintainer
+        if: steps.meta.outputs.update-type == 'version-update:semver-major'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          gh pr comment "${{ github.event.pull_request.html_url }}" \
+            --body "Major version bump, so this one waits for a maintainer."