chore: add workflow to auto-update major version tag on release

Adds update-major-tag.yml that triggers on any published release and
force-updates the major version tag (e.g., v6) to point to the latest
release (e.g., v6.0.1). Enables users to pin to @v6 instead of full
version numbers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/update-major-tag.yml

@@ -0,0 +1,23 @@
+name: Update major version tag
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  update-tag:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+      - name: Update major version tag
+        run: |
+          VERSION="${{ github.event.release.tag_name }}"
+          MAJOR="v$(echo "$VERSION" | sed 's/^v//' | cut -d. -f1)"
+          echo "Updating $MAJOR tag to point to $VERSION"
+          git tag -f "$MAJOR" "$VERSION"
+          git push -f origin "$MAJOR"