docs: define standalone Agents API boundaries

[chubes4]

Summary

• Rewrites the README around the standalone Agents API substrate boundary.
• Clarifies ownership across Abilities API, wp-ai-client, Agents API, and product consumers such as Data Machine.
• Adds consumer guidance for dependency handling and feature detection while keeping v1 backend-only.

Tests

• composer test
• Manually verified README mentions Data Machine only as an example consumer/proving ground, not as a dependency or mirrored structure.

Closes #1

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting and validating standalone Agents API boundary documentation under Chris's review.