docs: propose MetaHarness assurance companion

[pacphi]

Summary

• replace the branch-only MetaHarness retrieval direction with an optional assurance-companion proposal
• add Proposed ADR-0022 with explicit external companion and thin read-only projection boundaries
• document the file-by-file disposition of commit 8dcad80 and the retirement of its retrieval-improvement DDD
• keep all runtime integration, result-schema, command, routing, key, and promotion work unimplemented

Status

Documentation only. ADR-0022 remains Proposed; merging this PR does not mark it Accepted or Implemented.

Superseded branch

This PR replaces the unique MetaHarness documentation commit 8dcad80 on docs/metaharness-integration-proposal. That branch should be deleted only after this PR merges and a final dependency check passes.

Validation

• pnpm run check
• pnpm run lint:links:internal
• pnpm run lint:links
• git diff --check