docs: define standalone Agents API boundaries

[chubes4]

Goal

Rewrite Automattic/agents-api docs so the repository describes itself as a standalone generic WordPress agent substrate, not as a Data Machine extraction artifact.

Boundary to document

Abilities API  -> actions and tools
wp-ai-client   -> provider/model prompt execution
Agents API     -> durable agent runtime substrate
Data Machine   -> automation product consumer

Required decisions to make explicit

• Agents API must not know about Data Machine.
• Data Machine can require Agents API because Data Machine is a product consumer.
• Agents API owns generic contracts and value objects: registration, runtime messages/results, packages, memory, transcripts, tool declarations.
• Data Machine owns flows, pipelines, jobs, handlers, queues, retention, content operations, admin UI, and product CLI.
• wp-ai-client is the provider/model execution layer; Agents API should not reintroduce provider-specific request code.
• v1 remains backend-only: no admin UI and no public REST controllers unless separately designed.

Desired output

• README is concise and consumer-facing.
• Add a small architecture/boundary doc if README would get too dense.
• Add a migration/consumer note explaining how product plugins should depend on and feature-detect Agents API.
• Remove stale extraction phrasing that makes Agents API sound like a Data Machine submodule.

Acceptance criteria

• Docs mention Data Machine only as an example consumer/proving ground, not as something Agents API imports or mirrors.
• Docs include the layer diagram above or a better equivalent.
• composer test still passes.
• PR body includes AI assistance disclosure.