Move openai to [ai] extra; drop keyring/cryptography from core deps

[RFingAdam]

After the v5.0.0 AI removal, openai/keyring/cryptography remained mandatory deps but are imported nowhere (verified) — ~50 MB of dead transitive deps. Moved openai to the ai extra; removed keyring/cryptography.

Acceptance criteria

• Core install no longer pulls openai/keyring/cryptography. (Done — closed by the accompanying commit.)

Ref: pyproject.toml, requirements.txt. (R15)