Prepare @workit/core 0.4.0 contract hardening release

[AdmilsonCossa]

Summary

Prepare @workit/core 0.4.0 as a runtime contract and evidence hardening release.

• add @workit/core/contracts for declared cancellable/shielded task composition
• add @workit/core/fault for bounded in-process lifecycle fault scenarios
• add declared agent tool capability checks in @workit/core/ai
• extend @workit/core/ledger with caller-owned SQLite/Postgres receipt ledger adapters
• update API/package-consumer coverage, evidence, docs, changelog, and version metadata

Boundaries

• root @workit/core import remains unchanged
• contracts is a compile-time intent contract, not proof that arbitrary task bodies observe ctx.signal
• fault is an in-process evidence harness, not OS/process/network chaos testing or deterministic replay
• SQL ledger adapters define caller-owned storage ports, not a database framework

Validation

• npm run check:headers
• npm run check:security
• npm run check:vulnerabilities
• npm run test:coverage — 27 files, 354 tests, 100% statements/branches/functions/lines
• npm run test:evidence — 22 proof files, 0 failed
• npm run check:api
• npm run check:size — public API 29,255 B minified / 9,694 B gzip
• npm run check:package-consumer
• npm run verify
• npm run check:release