Switch default OpenAI model to gpt-5.4-mini

[timrogers]

Motivation

• Update the default OpenAI chat-completions model to gpt-5.4-mini so the CLI uses the newer model for LLM-powered inference.

Description

• Change the OPENAI_MODEL constant in src/llm.rs from gpt-4o to gpt-5.4-mini.

Testing

• Ran cargo fmt --all -- --check, which completed successfully.
• Ran git diff --check, which completed successfully.
• pre-commit run --all-files was not run in this environment because pre-commit is not installed.
• cargo test --locked --all-features and cargo build --release --locked were not executed due to the local Rust toolchain being older than the crate MSRV (installed rustc 1.89.0 vs required 1.94).

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Codex Task