feat(platform-knowledge): add cross-service design patterns guidance

[scotwells]

Summary

• Adds platform-knowledge/cross-service-patterns.md documenting six architectural principles for how services in the Datum Cloud platform should relate to each other
• Updates platform-knowledge/SKILL.md to reference the new file
• Updates plan agent to load it when a feature touches more than one service
• Updates api-dev agent to load it when projecting resources into another service's API

Principles documented

• Directed dependency — producers push into downstream APIs; downstream services don't watch upstream ones
• Resource projection — cross-service configuration is projected as explicit CRs in the downstream API
• Declaration vs. evaluation — config resources declare intent; runtime evaluation is a separate system's concern
• Status conditions reflect config health only — not runtime compliance or live system state
• Inline vs. separate CRD — inline when a concept is inherently per-resource; CRD only when shared lifecycle is needed
• Technology-neutral APIs — backend translation is a deployment concern, not an API concern

How agents consume it

The plan agent loads it during context discovery whenever the feature spans multiple services. The api-dev agent loads it from the task table when implementing cross-service resource projection.