Document isolated test-only CI adoption

[FU-max-boop]

Summary

• add a copyable real-Runner contract test for downstream adapters
• document an isolated uv matrix for exact OpenAI Agents SDK 0.20.0 and 0.21.0
• surface immutable release hashes, provider-free limits, and the remaining third-party dependency trust boundary

This keeps AgentRunProof out of downstream runtime metadata and normal lock resolution while giving maintainers a focused compatibility lane.

Validation

• exact SDK 0.20.0: 2 passed
• exact SDK 0.21.0: 2 passed
• downstream --with-editable . shape: 2 passed
• full suite: 170 passed, 1 skipped
• Ruff check/format, mypy, build, Twine strict, YAML parse, and git diff --check
• documented wheel/sdist hashes match the immutable v0.2.0 release