Document integrations architecture layers and provider contribution rules

[slombardo]

• Add/extend architecture docs (create docs/IntegrationsArchitecture.md or equivalent)
• Diagram allowed project references (Integrations <-> Providers, no cross to Models/Services/etc.)
• Describe process to add a new provider (entry points, contracts, boundaries)
• Call out strict “no domain/data/service access” in Providers, even for advanced/complex integrations
• Reference this doc as pre-req for community PRs involving integrations code

Exit criteria:

• Clear architecture diagram in doc
• Provider contribution steps & restrictions
• Doc linked from main documentation index or README