`codex` model alias resolves to non-existent model IDs

[ellisjr]

Description

Using --model codex fails on both routing paths:

• OpenRouter: Resolves to openrouter/openai/gpt-5.3-codex — model does not exist
• Direct OpenAI API: openai/codex-mini-latest — model does not exist

Steps to Reproduce

# Via OpenRouter alias
sidecar start --model codex --no-ui --prompt "Hello"
# Error: "This request requires more credits, or fewer max_tokens"
# (misleading — actual issue is model doesn't exist on OpenRouter)

# Via direct API
sidecar start --model openai/codex-mini-latest --no-ui --prompt "Hello"
# Error: "The requested model 'codex-mini-latest' does not exist."

Expected Behavior

codex alias maps to a valid, available model — or fails fast with a clear error message like Error: 'codex' alias is not configured or maps to an unavailable model.

Actual Behavior

Silently resolves to non-existent model IDs, producing confusing error messages (especially via OpenRouter where the credit error masks the real issue).

Suggestion

Either update the alias mapping to a valid model, or add a --validate-model check that verifies the resolved model exists before launching a session.