When _ensure_runtime_credentials() resolves the provider to openai-codex,
check if the active model is Codex-compatible. If not (e.g. the default
anthropic/claude-opus-4.6), swap it for the best available Codex model.
Also strips provider prefixes the Codex API rejects (openai/gpt-5.3-codex
→ gpt-5.3-codex).
Adds _model_is_default flag so warnings are only shown when the user
explicitly chose an incompatible model (not when it's the config default).
Fixes#651.
Co-inspired-by: stablegenius49 (PR #661)
Co-inspired-by: teyrebaz33 (PR #696)
