fix(schema): reject the direction-inverting documented_by alias

[xiehuanyi]

Summary

Stop converting file --documented_by--> document into file --documents--> document, which reverses the relationship. Following the existing implemented_by/tests policy, leave this inverse alias unnormalized so validation drops it with an invalid-edge issue; canonical documents edges retain their original endpoints.

Linked issue(s)

Closes #653.

How I tested this

Added regressions for reporting/dropping documented_by and preserving canonical documents edges. Core build also passes.

• pnpm lint
• pnpm --filter @understand-anything/core test — 1,015 passing
• pnpm test — 786 passing, 12 skipped
• Manual smoke test — covered by the deterministic graph validation regression

Versioning

• Version bumped to 2.9.8 in all six manifests listed in CLAUDE.md

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