Share general review and documentation rules

General error, cancellation, test, and documentation rules keep
contracts precise without importing another project's package layout.
Multiline boundaries, real-path tests for text-dependent errors,
required test dependencies, and support claims have explicit rules.

Signature verification can use a temporary allowed-signers file without
configuration changes. Discussion-derived changes include an archive
link. The existing code placement and signing exception stay intact.

Issue: 3cdc678