fix: require local-check artifacts to exist on disk

[Frank-zhu0404]

Summary

Closes #21.

validate-coverage-ledger.cjs previously only checked that local_checks[].artifact paths were lexically owned (agents/<agent_id>/artifacts/...). A covered / blocked / candidate unit could therefore pass while pointing at evidence that was never retained.

This change makes the CLI resolve each local-check artifact against the ledger directory (<output-dir>/coverage-ledger.json) and require that it:

• exists on disk
• is a regular file (lstat, not a directory / FIFO / etc.)
• is not a symlink
• stays inside the owning agent's real artifacts directory after realpath

The same checks apply to archived attempts[].local_checks[]. Programmatic validateDocument(ledger) without a base directory stays schema-only so existing in-memory unit tests keep working; pass baseDir (as the CLI does) to enable on-disk checks.

Test plan

• node --test skills/security-audit/validate-coverage-ledger.test.cjs (37 pass)
• node --test skills/security-audit/validate-findings.test.cjs (34 pass)
• Manual repro from Coverage validator accepts nonexistent local-check artifacts #21 now exits nonzero with:
  $[0].local_checks[0].artifact: local check artifact does not exist
• New coverage for missing artifact, directory artifact, symlink artifact, archived missing artifact, retained regular file, and schema-only vs baseDir behavior