Switch default OpenAI model to gpt-5.4-mini

[timrogers]

Motivation

• Use a newer OpenAI chat-completions model by default, switching the configured model from gpt-4o to gpt-5.4-mini.

Description

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

Testing

• Ran git diff --check (clean), attempted pre-commit run --all-files (failed because pre-commit is not installed), and attempted cargo test --locked --all-features and cargo build --release --locked (both failed because the local rustc 1.89.0 is below the repository MSRV 1.94).

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Codex Task

[Copilot]

Pull request overview

This PR updates the OpenAI chat-completions default model used by the formanator LLM integration by changing the OPENAI_MODEL constant in src/llm.rs.

Changes:

• Switch the default OpenAI model from gpt-4o to gpt-5.4-mini.

Show a summary per file

File	Description
src/llm.rs	Updates the hard-coded default OpenAI model string used for chat-completions requests.

Copilot's findings

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[timrogers]

@copilot Break a test in this PR

[Copilot]

@copilot Break a test in this PR

Added a targeted unit test in src/llm.rs that asserts the legacy default OpenAI model value (gpt-4o) and confirmed it fails against the current implementation. Commit: 18f15d5.