Part of #2134 — Phase 4.

Spec: .local/specs/022-config-simplification/spec.md § Phase 5 (Env Override Update)

Goal

Update ZEPH_LLM_* environment variable overrides to work with the new [[llm.providers]] schema.

Tasks

Update crates/zeph-config/src/env.rs:

• ZEPH_LLM_PROVIDER → sets providers[0].type (error if >1 provider and ambiguous)
• ZEPH_LLM_MODEL → sets providers[0].model
• ZEPH_LLM_BASE_URL → sets providers[0].base_url
• Remove env vars that mapped to now-deleted top-level fields

Document updated env vars in docs/.

Acceptance

• All env-override tests pass
• ZEPH_LLM_MODEL overrides the default provider model in a multi-provider config
• Env override tests cover both single-provider and multi-provider cases