fix(agent): pass dummy apiKey for Ollama provider registration

Pi SDK's registerProvider requires apiKey even for Ollama's
OpenAI-compatible API. Pass 'ollama' as a placeholder — Ollama
ignores it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/gateway/src/agent/provider.service.ts

@@ -96,6 +96,7 @@ export class ProviderService implements OnModuleInit {
       id: 'ollama',
       name: 'Ollama',
       baseUrl: `${ollamaUrl}/v1`,
+      apiKey: 'ollama',
       models: modelIds.map((id) => ({
         id,
         name: id,