Add playground package.json and include in workspace

Editor assessment

The subject states both parts of the change exactly — a playground package.json is added and the workspace includes it — so the header communicates without the diff, and chore is a fair classification for repository scaffolding. Hygiene is clean: imperative mood, no trailing period, sensible length, no ticket junk. No body is needed for a small structural addition, and no footer is required. A note on why the playground needed its own manifest would add a little value, but the message is already clear, accurate, and publishable in the good band.