Fix unsupported Codex OAuth default model in init

[morluto]

Summary

Updates the OpenAI Codex OAuth default model from openai-codex/gpt-5.3-codex to openai-codex/gpt-5.4.

gpt-5.3-codex is no longer supported when using Codex with a ChatGPT account, so the previous default caused Codex OAuth runs initialized through Vibe-Trading to fail with:

The 'gpt-5.3-codex' model is not supported when using Codex with a ChatGPT account.

Changes

• Replaces the Codex OAuth default model in provider metadata and legacy init config.
• Updates .env.example to use openai-codex/gpt-5.4.
• Adds OpenAI Codex to the newer vibe-trading init onboarding provider list.
• Shows Codex-specific OAuth guidance in onboarding instead of treating all no-key providers as Ollama.
• Updates/adds regression tests for Codex defaults and init behavior.

Related Issue

Fixes #445

Verification

.venv/bin/python -m pytest \
  agent/tests/test_openai_codex.py \
  agent/tests/test_cli_init.py \
  agent/tests/test_llm_provider_defaults.py \
  agent/tests/test_cli_interactive.py

87 passed

[warren618]

Thanks @morluto. I integrated the Codex OAuth default model update into main in 04ccb21, including onboarding/provider default coverage for openai-codex/gpt-5.4. You are credited in the commit via Co-authored-by. Closing this PR because the equivalent change is now on main.