CI: add reusable Docker Hub description workflow - #6

Adds .github/workflows/dockerhub-description.yml, a reusable workflow that syncs the Docker Hub README and short description for an image.

• Short description is read from the GitHub repo description (via API, so scheduled runs work too), trimmed to 100 chars — GitHub becomes the single source of truth.
• Inputs: image (required), readme (default ./README.md). Uses DOCKERHUB_USERNAME / DOCKERHUB_TOKEN via secrets: inherit.

Previously the per-repo peter-evans/dockerhub-description steps never passed short-description, so all Hub short descriptions drifted from GitHub.

⚠️ Merge this first — follow-up PRs across dockette repos call it at @master.