[Dev Server] Add `next internal apply` for agentic patch submission
When `experimental.agenticEditing` is enabled, disable Turbopack's native
filesystem watcher and expose a `POST /_next/dev/apply` endpoint as the
sole compilation trigger. The agent submits a search/replace patch + URLs,
the endpoint writes files, calls `invalidateFileSystemCache()`, waits for
compilation to finish, and returns errors/screenshots in one call.