chore: add .nvmrc and auto-tag workflow on PR merge

- .nvmrc pins Node 20.12.0 for consistent contributor environment
- release.yml: when a PR merges to main, auto-create v{version} tag
  which triggers the existing publish.yml → npm publish pipeline

Author: m1ngsama

.github/workflows/release.yml

@@ -0,0 +1,40 @@
+name: Auto Tag on PR Merge
+
+on:
+  pull_request:
+    types: [closed]
+    branches: [main]
+
+jobs:
+  tag:
+    if: github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - name: 🛎️ Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: 📋 Read version from package.json
+        id: version
+        run: echo "version=$(node -p "require('./package.json').version")" >> $GITHUB_OUTPUT
+
+      - name: 🔍 Check if tag already exists
+        id: tag_check
+        run: |
+          if git rev-parse "v${{ steps.version.outputs.version }}" >/dev/null 2>&1; then
+            echo "exists=true" >> $GITHUB_OUTPUT
+          else
+            echo "exists=false" >> $GITHUB_OUTPUT
+          fi
+
+      - name: 🏷️ Create and push tag
+        if: steps.tag_check.outputs.exists == 'false'
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git tag "v${{ steps.version.outputs.version }}"
+          git push origin "v${{ steps.version.outputs.version }}"