chore: scaffold project tooling and CI

[Apetuezekiel]

What

Sets up the complete project skeleton for tiergraph — a zero-runtime-dependency TypeScript library for SaaS tier-based feature gating.

Files added:

• package.json — name tiergraph, version 0.0.0, MIT, ESM+CJS dual exports (import/require conditions), files field restricting publish to dist/, schema/, README.md, LICENSE
• tsconfig.json — strict, ES2022 target, NodeNext module resolution, exactOptionalPropertyTypes, noUncheckedIndexedAccess
• tsup.config.ts — dual ESM+CJS format, dts, sourcemaps, treeshake, no splitting
• vitest.config.ts — 100% coverage thresholds on src/ via v8 provider
• biome.json — recommended ruleset, tab indentation, single quotes, trailing commas
• .github/workflows/ci.yml — lint → typecheck → test → build on Node 18 and 20, triggers on push and PR
• README.md — stub (one paragraph, "under construction")
• src/index.ts — empty export so build doesn't fail
• src/index.test.ts — placeholder test that imports the module (satisfies coverage threshold on the stub)

Why

Establishes the tooling baseline required by the v1 scope: TypeScript strict mode, tsup for dual publish, vitest for 100% coverage enforcement, biome for lint/format, and GitHub Actions CI. Every subsequent PR builds on top of this foundation.

How to verify

npm install
npm run lint       # biome: no errors
npm run typecheck  # tsc --noEmit: clean
npm run test       # vitest: 1 test passing, coverage thresholds met
npm run build      # tsup: dist/index.js, dist/index.cjs, dist/index.d.ts emitted

Out of scope

• No source logic yet (defineConfig, createAccess, error classes) — those are PR 2 and PR 3
• No JSON Schema generation — that is PR 4
• Full README — that is PR 5