The Codex model normalization was rejecting any model without 'codex'
in its name, forcing a fallback to gpt-5.3-codex. This blocked models
like gpt-5.4 that the Codex API actually supports.
The fix simplifies _normalize_model_for_provider() to two operations:
1. Strip provider prefixes (API needs bare slugs)
2. Replace the *untouched default* model with a Codex-compatible one
If the user explicitly chose a model — any model — we trust them and
let the API be the judge. No allowlists, no slug checks.
Also removes the 'codex not in slug' filter from _read_cache_models()
so the local cache preserves all API-available models.
Inspired by OpenClaw's approach which explicitly lists non-codex models
(gpt-5.4, gpt-5.2) as valid Codex models.
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)
