[P0] Integration: Custom GPT Actions (OpenAPI spec)

Build OpenAPI specification and GPT instructions for ChatGPT to access ChittyOS services via Custom GPT Actions.

**Deliverables:**
- OpenAPI 3.0 spec for ChittyOS API
- GPT instructions markdown
- Authentication setup
- Testing documentation

**Priority:** P0
**Status:** Planned
**Project:** ChittyOS Integrations Suite