Migrate from lerna to npm workspaces

[bobur-khay]

resolves #379

This repo uses Lerna v5 (CI pins v6.6.2) purely as a lightweight bootstrap/task-runner, no advanced features like lerna publish, lerna version, lerna changed, or lerna diff are used anywhere. The only two Lerna commands in the entire codebase are:

Lerna command	npm workspaces equivalent
lerna bootstrap	npm install (automatic with workspaces)
lerna run --ignore @thing-description-playground/web test	npm test --workspaces --if-present + exclude web from workspaces, or npm test -w packages/core -w packages/assertions -w packages/cli -w packages/defaults

Lerna is pure overhead here, it adds a global install step to CI, an extra config file, and a dependency, all to do what npm workspaces does natively since npm 7+ (Node 16+).

[netlify]

✅ Deploy Preview for thingweb-playground ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	1c9911e
🔍 Latest deploy log	https://app.netlify.com/projects/thingweb-playground/deploys/6a16b10b0ad4990008176f50
😎 Deploy Preview	https://deploy-preview-720--thingweb-playground.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 24 (no change from production) Accessibility: 79 (no change from production) Best Practices: 100 (no change from production) SEO: 80 (no change from production) PWA: 70 (no change from production) View the detailed breakdown and full score reports
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.