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>

pnpm-lock.yaml

@@ -41,6 +41,9 @@ importers:
 
   apps/gateway:
     dependencies:
+      '@mariozechner/pi-ai':
+        specifier: ~0.57.1
+        version: 0.57.1(ws@8.19.0)(zod@4.3.6)
       '@mariozechner/pi-coding-agent':
         specifier: ~0.57.1
         version: 0.57.1(ws@8.19.0)(zod@4.3.6)
@@ -53,6 +56,9 @@ importers:
       '@mosaic/db':
         specifier: workspace:^
         version: link:../../packages/db
+      '@mosaic/types':
+        specifier: workspace:^
+        version: link:../../packages/types
       '@nestjs/common':
         specifier: ^11.0.0
         version: 11.1.16(class-transformer@0.5.1)(class-validator@0.15.1)(reflect-metadata@0.2.2)(rxjs@7.8.2)