[Task] Add LLM context documentation (alpha-only)

[Rexus]

Task

Add a dedicated context documentation file to help agentic LLMs understand the purpose, structure, and goals of Archflow.

This file is intended for use during alpha development only and should provide clear guidance for AI-assisted development.

---

Done when

• A context document exists (e.g. docs/context.md or AGENTS.md)
• It describes Archflow goals, architecture, and design principles
• It explains YAML-first approach and projection model
• It is easy to reference from prompts or tooling