Document registry resolution paths for agent readers

[adohe]

Spec candidate: #3.

Scope

• Explain how algorithm registration and model adapter registration work.
• Show how an agent can discover built-in algorithms and model adapters.
• Explain why AReno keeps registries and self-contained runtime ownership instead of copying pith-train's direct instantiation style.

Acceptance

• Preserves the existing "register, don't branch" AReno extension rule.
• Cites current registry files and public discovery helpers.
• No public API changes.