Make consumer boundary product-agnostic

[t3chn]

Closes #7.

Summary:

• Replace product-specific boundary language with product-agnostic consumer application language.
• Rename the scope/boundary doc to canonical-scope-and-consumer-boundary.
• Add the three-boundary model: Agent Bench Lab, Private Eval Layer, Consumer Application.

Test plan:

• make validate
• make test
• make leak-check
• python3 -m ruff check .
• git diff --check