feat: add JSON Schema export for config validation

[Apetuezekiel]

What

Generates and publishes a JSON Schema (schema/config.schema.json) derived from the ConfigInput TypeScript type, enabling config validation in non-TypeScript runtimes.

Changes:

• schema/config.schema.json — checked-in JSON Schema (draft-07) for ConfigInput. Covers plans, PlanInput (features, extends, limits), Limit (quota: number | null, period enum), and Period.
• package.json — adds schema:generate script using ts-json-schema-generator; ts-json-schema-generator added as devDependency. schema/ already in files field from PR 1.
• .github/workflows/ci.yml — adds "Check schema is up to date" step: regenerates the schema and fails the job with an explanatory message if the committed file differs from the generated output.

Why

Implements v1 scope item 8. The JSON Schema lets non-TypeScript callers (Python services, CLI tools, config validators) validate a tiergraph config at runtime without shipping the TypeScript compiler. The CI drift check prevents the schema from silently going stale when types evolve.

How to verify

# Schema validates correctly against a sample config
npm run schema:generate   # should produce identical output (git diff clean)
cat schema/config.schema.json  # inspect structure

# CI drift check: mutate a type, regenerate, confirm CI would fail
# git diff schema/config.schema.json  # shows diff → CI step would exit 1

Out of scope

• Full README — PR 5
• Runtime schema validation helper — not in v1 scope; users wire the schema to their preferred JSON Schema validator