ci: add auto-tag workflow missing from main (#153)

## Summary

- Adds `auto-tag.yml` workflow that was created during v0.9.0
development but missed the squash merge to main
- Without this workflow, release PRs don't trigger automatic tag
creation, breaking the release pipeline

## Context

The release pipeline chain is: `release/* PR merge → auto-tag creates
vX.Y.Z → release.yml builds + publishes`

This workflow was in `release/0.9.0` but the squash merge of PR #149
didn't include it, so v0.9.0 never got tagged or released.

## After merge

Once this is on main, I'll manually create the `v0.9.0` tag to trigger
the release build.

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

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: tomymaritano

.github/workflows/auto-tag.yml

@@ -0,0 +1,59 @@
+name: Auto Tag & Sync
+
+on:
+  pull_request:
+    types: [closed]
+    branches: [main]
+
+permissions:
+  contents: write
+
+jobs:
+  tag-and-sync:
+    if: github.event.pull_request.merged == true && startsWith(github.event.pull_request.head.ref, 'release/')
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout main
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          token: ${{ secrets.GH_TOKEN }}
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Read version from package.json
+        id: version
+        run: |
+          VERSION=$(node -p "require('./package.json').version")
+          echo "version=${VERSION}" >> "$GITHUB_OUTPUT"
+          echo "tag=v${VERSION}" >> "$GITHUB_OUTPUT"
+
+      - name: Check tag doesn't already exist
+        id: check
+        env:
+          TAG_VERSION: ${{ steps.version.outputs.version }}
+        run: |
+          if git rev-parse "v${TAG_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.check.outputs.exists == 'false'
+        env:
+          TAG_NAME: ${{ steps.version.outputs.tag }}
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git tag -a "${TAG_NAME}" -m "Release ${TAG_NAME}"
+          git push origin "${TAG_NAME}"
+
+      - name: Merge main → develop
+        run: |
+          git fetch origin develop
+          git checkout develop
+          git merge main --no-edit
+          git push origin develop