Parent epic: #2141

Phase 2: Config + bootstrap

Tasks

• Add ComplexityRoutingConfig struct to zeph-config/src/providers.rs:
  • enabled: bool, triage_provider: Option<String>, bypass_single_provider: bool
  • ComplexityTierMapping with simple/medium/complex/expert: Option<String> (provider names)
• Add LlmRoutingStrategy::Triage variant
• Bootstrap: resolve triage_provider name to ProviderEntry from pool, build via build_provider_from_entry()
• Config validation: triage_provider must reference an existing pool entry
• --migrate-config support (no migration needed from legacy — new feature)
• Add [llm.complexity_routing] section to config/default.toml (disabled by default)

Key files

• crates/zeph-config/src/providers.rs
• crates/zeph-core/src/bootstrap/provider.rs
• config/default.toml

Acceptance criteria

• Config round-trips through TOML serialize/deserialize
• Validation rejects nonexistent triage_provider name
• bypass_single_provider = true skips triage when pool has 1 entry