Introduces the ports-and-adapters standard new backend modules are
expected to follow, plus the two pieces that make it more than prose:
`deerflow/domain/` as the inner-ring namespace, and an AST test that
fails when anything under it imports infrastructure.
The spec is normative rather than descriptive -- the existing modules
predate it, so the test guards the namespace, not the whole backend.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
