ci: enable Dependabot with auto-merge for patch & minor updates (#148)

Adds .github/dependabot.yml (weekly npm + github-actions; minor/patch
grouped) and a dependabot-auto-merge workflow that enables GitHub
auto-merge (gh pr merge --auto --squash) for patch & minor bumps, leaving
major versions for manual review. Uses fetch-metadata; PR URL passed via
a quoted env var (no injection); does not check out PR code.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,24 @@
+version: 2
+updates:
+  # npm dependencies — group low-risk bumps so auto-merge handles them in one PR.
+  - package-ecosystem: npm
+    directory: "/"
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 10
+    labels:
+      - dependencies
+    groups:
+      minor-and-patch:
+        update-types:
+          - minor
+          - patch
+
+  # Keep GitHub Actions pinned and current.
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly
+    labels:
+      - dependencies
+      - github-actions

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,33 @@
+name: Dependabot auto-merge
+
+# Auto-merge Dependabot PRs for low-risk updates (patch + minor, and any
+# security patch). Major version bumps are left for manual review.
+#
+# Requires "Allow auto-merge" enabled on the repo. `gh pr merge --auto` queues
+# the merge so it only lands once required status checks pass.
+
+on: pull_request_target
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Fetch Dependabot metadata
+        id: meta
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Enable auto-merge for patch & minor updates
+        if: >-
+          steps.meta.outputs.update-type == 'version-update:semver-patch' ||
+          steps.meta.outputs.update-type == 'version-update:semver-minor'
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}