feat(cli): prefer configured startup provider and openai defaults

[shuyhere]

Summary

• honor an explicitly configured default provider/model before heuristic startup fallback
• prefer OpenAI/OpenAI Codex when authenticated so startup defaults align with the current global fallback model
• update default provider/model parsing to use OpenAI / gpt-5.4 by default

Why

This separates startup model selection behavior into a small reviewable change instead of bundling it with fullscreen and compaction work.

Files

• crates/cli/src/login/resolver/models.rs
• crates/core/src/agent_session/model_arg.rs