Preserve gh auth in fake Responses harness

[cbusillo]

Summary

• split harness auth inheritance into Code auth and GitHub CLI auth paths
• keep suppressing Code auth when fake Responses is enabled
• continue copying GitHub CLI auth when inherit_auth is requested
• expose split auth state in the run manifest

Validation

• python3 -m py_compile tools/code-exec-harness/harness.py
• poisoned fake Responses harness run with parent Code auth, parent gh config, and parent OPENAI_API_KEY; verified Code auth was not copied, gh config was copied, and 2 fake Responses requests were captured
• ./build-fast.sh