fix(devkit): roll back thenv mini app and shell to scaffold (#272)

## Summary
- roll back the Devkit thenv mini app from live implementation to
placeholder scaffold
- remove Devkit-local thenv UI/api client and /api/thenv/* proxy
routes/tests
- revert Devkit shell to the initial bootstrap shell (simple header +
inline navigation)
- align mini-app registry/tests/home-page copy so all mini apps are
placeholder
- update Devkit/Thenv contract docs to reflect scaffold-only state and
deferred integrations

## Testing
- `pnpm test` (in `apps/devkit`)

Author: kdy1