docs: choose the right context layer

[EmersonBraun]

Summary

• explain the distinct roles of AGENTS.md, Doc Bridge, code search, RAG, and human review
• document the recommended context order for coding agents
• clarify that editRoots is a routing and audit contract, not an OS sandbox
• add the guide to the official docs navigation

Validation

• pnpm typecheck
• pnpm build
• pnpm docs:typecheck
• pnpm check:docs
• pnpm test (189/190 pass locally; one existing environment-sensitive Ollama error-path assertion sees a running Ollama instance without nomic-embed-text)

This is a docs-only change and does not alter runtime behavior.