Context

PR implementing #2444 (cost-sensitive store routing) added StoreRoutingConfig, LlmRouter, HybridRouter, and the AsyncMemoryRouter trait. However, context_manager.rs::build_router() still always returns HeuristicRouter regardless of the [memory.store_routing] config.

Required Change

In zeph-core/src/agent/context_manager.rs, update build_router() to read memory.store_routing config and return the appropriate router:

• strategy = "heuristic" → HeuristicRouter (current behavior)
• strategy = "llm" → LlmRouter constructed with the configured routing_classifier_provider
• strategy = "hybrid" → HybridRouter with configured provider and confidence_threshold

The router is Arc<dyn MemoryRouter> — async callers in the recall path should downcast to AsyncMemoryRouter when LLM routing is active.

Acceptance Criteria

• build_router() respects [memory.store_routing] strategy
• strategy = "heuristic" is identical to current behavior
• LLM and hybrid strategies call the configured provider
• Fallback to heuristic on LLM failure preserved