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/src/chat/chat.gateway.ts

@@ -17,6 +17,8 @@ import { v4 as uuid } from 'uuid';
 interface ChatMessage {
   conversationId?: string;
   content: string;
+  provider?: string;
+  modelId?: string;
 }
 
 @WebSocketGateway({
@@ -65,7 +67,10 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
     try {
       let agentSession = this.agentService.getSession(conversationId);
       if (!agentSession) {
-        agentSession = await this.agentService.createSession(conversationId);
+        agentSession = await this.agentService.createSession(conversationId, {
+          provider: data.provider,
+          modelId: data.modelId,
+        });
       }
     } catch (err) {
       this.logger.error(