P0: Initialize monorepo (pnpm workspaces + Turborepo)
Imported from GitHub issue El-SaMa/oma#1 by @El-SaMa.
Set up the monorepo skeleton: apps/web, services/core-api, services/mcp-server, services/workers, packages/db, packages/auth, packages/ui, packages/contracts. Configure workspace tooling and shared tsconfig.
Done when: install works, workspaces resolve, root scripts run across packages.