Function find_codex_binary 

pub fn find_codex_binary(explicit: Option<&Path>) -> Result<PathBuf, AppError>

Locates the Codex CLI binary on the system.

Search order:

1. Explicit --codex-binary CLI flag.
2. SQLITE_GRAPHRAG_CODEX_BINARY env var.
3. PATH search for codex (or codex.exe on Windows).