chore: onboard Dependabot + automerge workflow

Adds the standard ether/* dependabot.yml (npm + github-actions, daily,
versioning-strategy: increase) and the Dependabot Automerge workflow
(squash-merge once test-and-release CI passes).

Brings this plugin in line with the other ~85 plugins under ether/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    versioning-strategy: "increase"

.github/workflows/automerge.yml

@@ -0,0 +1,30 @@
+name: Dependabot Automerge
+permissions:
+  contents: write
+  pull-requests: write
+on:
+  workflow_run:
+    workflows:
+      - Node.js Package
+    types:
+      - completed
+
+jobs:
+  automerge:
+    if: >
+      github.event.workflow_run.conclusion == 'success' &&
+      github.event.workflow_run.event == 'push' &&
+      github.event.workflow_run.actor.login == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Automerge
+        uses: "pascalgn/automerge-action@v0.16.4"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          MERGE_METHOD: squash
+          MERGE_LABELS: ""
+          MERGE_RETRY_SLEEP: "100000"
+