feat: tiered LLM providers (fast/heavy) with multi-provider config

Adds tier-based LLM routing so fast local models (vLLM, Ollama) handle
structured tasks while cloud models (Gemini, Anthropic) are reserved for
heavy reasoning. Single-provider configs continue to work via fallback.

- Tier type + ProviderRegistry with assignTier/getProvider/fallback chain
- Multi-provider config format: { providers: [{ name, type, tier, ... }] }
- NamedProvider wrapper for multiple instances of same provider type
- Setup wizard: Simple (legacy) / Advanced (fast+heavy tiers) modes
- Status display: tiered view with /llm/providers endpoint
- Call sites use getProvider('fast') instead of getActive()
- Full backward compatibility with existing single-provider configs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/tests/commands/status.test.ts

@@ -26,6 +26,7 @@ function baseDeps(overrides?: Partial<StatusCommandDeps>): Partial<StatusCommand
     log,
     write,
     checkHealth: async () => true,
+    fetchProviders: async () => null,
     isTTY: false,
     ...overrides,
   };