Fix public assert typings

[V3RON]

Summary

• replace the public assert type aliasing from a default node:assert/strict import with typeof import(...) so consumers keep the full Node assert surface instead of degrading to any
• add a focused consumer-style TypeScript fixture that typechecks the built package export and guards both core assert methods and custom SkillGym assertion helpers

Testing

• pnpm exec vitest run test/public-api.test.ts
• pnpm exec tsc -p ./tsconfig.build.json && pnpm exec tsc -p ./test/fixtures/public-assert-types/tsconfig.json