feat: add versioned JSON check reports

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Summary

• add evidrift check --format json
• expose a versioned, deterministic report schema for CI and coding agents
• preserve the existing exit codes for pass/warning, contract mismatch, and evidence-integrity failure
• document the integration contract and CI usage
• add unit and end-to-end coverage for valid reports, drift, tampering, and invalid formats

Why

Plain text is useful for humans, but CI systems and agents need a stable machine interface. The JSON report intentionally omits timestamps and absolute repository paths so identical results serialize identically.

Validation

• npm run verify
• 66/66 Node tests passed
• smoke test passed
• self-check passed
• lint and typecheck passed again after rebasing onto current main