refactor: design OpenAPI schema generator architecture

[soulcodingmatt]

Task

Design the module structure and interfaces for the OpenAPI schema generator. Define how it integrates with the existing annotation processing pipeline.

Parent story: #12
Parent epic: #18

Details

• Define a new openapi package (or separate module) with clear boundaries
• Design the SchemaGenerator interface that receives DTO metadata and emits YAML
• Determine how to hook into the existing EquilibriumProcessor lifecycle (new orchestrator? processor option?)
• Document the architecture decision

Acceptance Criteria

• Architecture document or ADR exists describing the module structure
• Interface definitions are committed (may be empty implementations)
• Integration point with existing processor is identified and documented