Adopt Turborepo and a single root .env

[RATCHAW]

What

Adopt Turborepo for task orchestration/caching, move to a single root .env every package reads, and expose every common script from the root. A contributor can now clone → pnpm install → cp .env.example .env → pnpm dev with no per-package steps.

Changes

• turbo.json — build/typecheck/test/dev/run-eval/report all dependsOn: ["^build"], so packages/core builds before its dependents typecheck/run against its dist/ (this replaces the ordering CI enforced by step comments). dev + eval tasks are cache: false; globalDependencies covers .env and tsconfig.base.json.
• Root package.json — turbo devDependency + dev/build/test/typecheck and the eval scripts (run-eval/report/fetch-data/pin) all via turbo run.
• Single root .env — new root .env.example (provider keys, PORT, eval settings); deletes apps/playground/.env.example. Tiny loaders resolve the repo-root .env from import.meta.url (cwd-independent, works under Turbo): apps/playground/src/server/env.ts (imported first in index.ts) and packages/evals/src/env.ts (imported in run/report/fetch/pin). vite.config.ts points loadEnv/envDir at the repo root. Ambient/exported vars still win — loadEnvFile never overrides.
• Docs — root README gains a Development section; playground + evals READMEs point at the root .env and root scripts; CLAUDE.md notes "pnpm + Turborepo."

Verification

• pnpm build cold → core first; re-run → >>> FULL TURBO cache hit.
• pnpm typecheck + pnpm test from root: all 3 packages pass, core built automatically.
• Env loading confirmed from both the repo root and the package dir; ambient vars override the file; playground server + Vite PORT proxy both read the root .env.
• Fresh-checkout (dist/ absent): pnpm dev from root builds core via Turbo, then boots the server cleanly — no ERR_MODULE_NOT_FOUND.
• CI (ci.yml) needs no change — same script names.

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