[WIP] Add codex exec support as LLM provider in deepagents-cli

[Copilot]

• Inspect current CLI provider detection, credential checks, model creation, and help text paths
• Run existing lint/build/test commands to baseline current state before edits
• Implement minimal Codex provider support (credential detection, provider detection, default model fallback, model creation path)
• Add/update focused unit tests for Codex behavior and help-screen drift expectations
• Run targeted tests for modified areas and perform a quick manual CLI help verification
• Run required validation checks and finalize with a minimal scoped update