test: add core package coverage

[salim4n]

Summary

• Add dedicated package-level tests for @ignitionai/agent-trainer-core.
• Cover dataset creation/validation, agent input conversion, mock adapters, run-result normalization, score clamping and weighted averages through the public barrel export.
• Update readiness, audit, milestones and backlog docs so core tests are no longer marked missing.

Out of scope

• Public API changes
• Runtime schema validation
• Report serialization
• Provider calls
• CLI changes
• IgnitionRAG integration

Validation

• bun test packages/core/src/index.test.ts
• bun run lint
• bun run typecheck
• bun run test
• bun run build
• bun run pack:check
• git diff --check