ci: Add GitHub Actions CI and dependabot with auto-merge

- Add CI workflow: build and test on push/PR to main
- Add dependabot for weekly Go module updates (Thursday JST)
- Add auto-merge workflow for dependabot PRs
- Group all dependencies into single PR

✨ Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Sakura 🌸 <noreply@anthropic.com>

Author: high-moctane

.github/dependabot.yml

@@ -0,0 +1,14 @@
+version: 2
+updates:
+  - package-ecosystem: gomod
+    directory: /
+    schedule:
+      interval: weekly
+      day: thursday
+      timezone: Asia/Tokyo
+    commit-message:
+      prefix: "deps"
+    groups:
+      all-dependencies:
+        patterns:
+          - "*"

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      - name: Build
+        run: go build ./...
+
+      - name: Test
+        run: go test ./...

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

@@ -0,0 +1,24 @@
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+
+      - name: Enable auto-merge for Dependabot PRs
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}