Share general review and documentation rules

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