Parent epic: #2141

Phase 1: Core types + triage prompt

Tasks

• Add ComplexityTier enum (Simple, Medium, Complex, Expert) to zeph-llm/src/router/triage.rs
• Add TriageVerdict struct (tier, reason, large_context) with Deserialize + JsonSchema
• Implement TriageRouter struct with assess() method that sends a minimal prompt to a cheap model
• Use chat_typed::<TriageVerdict>() for structured JSON output
• Triage prompt: ~150 input tokens, includes user message + conversation stats (msg_count, total_tokens, tool_count)
• Unit tests with MockProvider

Key files

• crates/zeph-llm/src/router/triage.rs (new)
• crates/zeph-llm/src/router/mod.rs (re-export)

Acceptance criteria

• TriageRouter::assess() returns TriageVerdict from mock provider
• Handles provider errors gracefully (fallback to Medium tier)
• Proptest: assess() never panics on arbitrary input