Drop fabricated "example" models for codex/gemini

The codex/gemini model listers returned hardcoded "example" model ids that looked pickable but weren't guaranteed valid (gpt-5-codex could fail outright). Neither CLI has a model-enumeration command, so remove the listers entirely. The model picker then offers just Default + Custom for those agents (its existing no-models behavior), and --models shows "(no advertised models; pass any value your CLI accepts via --model)".

Claude keeps its real, valid aliases (opus/sonnet/haiku). Tests updated.