Function resolve_roles_from_env 

pub fn resolve_roles_from_env(
    spec_backend: Option<&str>,
    spec_model: Option<&str>,
    spec_coding_backend: Option<&str>,
    spec_coding_model: Option<&str>,
) -> Result<ModelRoles>

Resolve both model roles. Each role’s backend is selected by its backend value (a URL -> openai; the keywords claude/codex/opencode -> that subprocess CLI), so the planner and coder can run on different backends.

• planner: backend (or $SCX_FORGE_BACKEND), $SCX_FORGE_API_KEY, model.
• coding: coding_backend (or $SCX_FORGE_CODING_BACKEND, else the planner backend value), $SCX_FORGE_CODING_API_KEY (else $SCX_FORGE_API_KEY), coding_model (else model).