Part of #2134 — Phase 2.

Spec: .local/specs/022-config-simplification/spec.md § Phase 2

Goal

Implement migration tooling and make old-format configs fail loudly.

Tasks

• Add migrate_llm_to_providers() to crates/zeph-config/src/migrate.rs
  • Detects presence of provider key (old format)
  • Converts [llm.cloud], [llm.openai], [llm.orchestrator], [llm.router] to [[llm.providers]]
  • Creates .bak backup before overwriting
• Add detection of old-format keys in LlmConfig deserialization → clear error: "legacy [llm.cloud] format removed in v0.17.0 — run: zeph --migrate-config"
• Rewrite config/default.toml to new format
• Update --init wizard to generate new format

Acceptance

• --migrate-config on each .local/config/*.toml produces valid new-format TOML
• Old-format config prints actionable error on startup
• --init produces new-format config
• All tests green