feat: multi-provider support — Anthropic + Ollama (P2-002)

Add ProviderService wrapping Pi SDK's ModelRegistry for multi-provider
LLM support. Built-in providers (Anthropic, OpenAI, Google, xAI, etc.)
auto-discovered; Ollama registered via OLLAMA_BASE_URL env var;
custom providers via MOSAIC_CUSTOM_PROVIDERS JSON env var.

- ProviderService: wraps ModelRegistry, manages provider lifecycle
- ProvidersController: GET /api/providers, GET /api/providers/models
- AgentService: accepts provider/model params on session creation
- ChatGateway: passes optional provider/modelId from chat messages
- @mosaic/types: new provider/model type definitions

Closes #20

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

apps/gateway/package.json

@@ -12,10 +12,12 @@
     "test": "vitest run --passWithNoTests"
   },
   "dependencies": {
+    "@mariozechner/pi-ai": "~0.57.1",
     "@mariozechner/pi-coding-agent": "~0.57.1",
     "@mosaic/auth": "workspace:^",
     "@mosaic/brain": "workspace:^",
     "@mosaic/db": "workspace:^",
+    "@mosaic/types": "workspace:^",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",
     "@nestjs/platform-fastify": "^11.0.0",