Add openapi-compatible assist provider

[scccy]

Summary

• add openapi_compatible assist provider for assist drafting
• support assist base_url, api_key, and model name configuration
• add CLI flags: --assist-provider, --assist-base-url, --assist-api-key
• normalize relay-backed assist output into valid DomainSpec shapes
• add provider, CLI, and precedence test coverage

Verification

• conda run -n codex-work python -m unittest tests.test_entry_init_domain_assist tests.test_llm_core_provider tests.test_llm_client_param_precedence
• conda run -n codex-work python -m unittest discover -s tests -p 'test_*.py'
• manually verified assist flow against local relay on http://127.0.0.1:8317/v1 with model gpt-5.4