Remove all app-builder code

[nichochar]

Summary

This removes the old app-builder product from the repo and restores Srcbook to a notebook-only shape.

The goal here was not to roll the repo back to a pre-app commit. Instead, this keeps the notebook/runtime improvements that still matter and deletes the app-specific surface area across backend, shared types/protocols, frontend routes/UI, and
docs.

What Changed

• removed app-builder backend modules under packages/api/apps
• removed app-builder HTTP routes from packages/api/server/http.mts
• removed the app websocket channel from packages/api/server/ws.mts
• removed app-edit / app-generation AI pipeline and prompts
• removed app-specific shared schemas/types/history/feedback exports
• removed app routes, app clients, and app components from packages/web
• restored the home screen to notebook-only behavior
• updated README/product copy to describe Srcbook as a TypeScript notebook again
• removed app-template packaging and related unused dependencies

Data / Migration

This PR adds a migration that drops the apps table:

• packages/api/drizzle/0017_drop_apps.sql

This is intentional. The app-builder product is no longer part of Srcbook.

Behavior Changes

• /apps/* routes are intentionally removed
• old app-builder UI/workflows no longer exist
• the home screen only exposes notebook flows now
• notebook creation/import/open/run flows remain intact

Verification

Manually smoke-tested:

• home screen loads
• create/open notebooks
• run JavaScript cells
• run TypeScript cells
• notebook AI flow
• settings / secrets flow

Also verified with project checks (pnpm lint)

Notes

/apps/* 404s are expected and intentional. This repo no longer carries compatibility shims for the removed app-builder product.