chore: easier playground discovery

[dprevost-LMI]

While working on the playground app, I saw some easy stuff that can make it more self-discoverable.

Note: I understand now that most of my problem was related to pnpm 10+ not running some scripts anymore and having trouble with sqllite3 binding, so I downgraded.

---

Note

Adds build/start/clean scripts and updates CONTRIBUTING workflow with build/test steps and a pnpm 10 sqlite3 warning.

• Docs:
  • Update CONTRIBUTING.md workflow: separate change/build/test steps, add pnpm build and pnpm start usage with the playground app.
  • Add warning for pnpm 10+ sqlite3 native binding errors (temporary workaround in apps/playground/metro.config.js).
• Tooling:
  • Add root package.json scripts: build (nx run-many -t build), start (nx start playground), and clean (reset and remove node_modules/dist).

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[V3RON]

Thank you for bringing this matter to my attention!
I have revised the contribution guide in #146, taking your changes into consideration. Therefore, I am closing this PR in favor of the linked one.