feat: OpenAPI specification extractor with domain model edges

Extracts structured relationships from OpenAPI 3.x specs. Direct
$ref resolution with one-level allOf/oneOf/anyOf expansion. Two-pass
domain model extraction: first identifies domain schemas via ubiquity
filter, then walks property-level $ref between domain schemas to
produce cross-domain edges (invoice → payment_intent → customer).

Validated on Stripe production spec (587 operations, 1359 schemas):
2,051 relationships in 11ms, 250 domain model edges, zero OOM.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: salmad3