docs: capture Claude Code test framework contracts

[bingran-you]

Summary

• add a cross-cutting verification-and-native-test-oracles subdomain for the observed Claude Code test framework
• document test posture, VCR fixtures, CI fail-closed policy, reset hooks, harness seams, and evidence boundaries
• add domain-owned contracts for settings schema compatibility and MCP federated-auth conformance, then link existing native-test-derived leaves back into the framework

Verification

• npx -p first-tree first-tree verify