feat(M3-003): implement OpenAI provider adapter for Codex gpt-5.4

Adds OpenAIAdapter implementing IProviderAdapter, replacing the legacy
registerOpenAIProvider() inline registration in ProviderService.

- Creates apps/gateway/src/agent/adapters/openai.adapter.ts with:
  - register(): initialises OpenAI client, registers codex-gpt-5-4 with
    Pi ModelRegistry using openai-completions API; skips gracefully when
    OPENAI_API_KEY is absent
  - listModels(): returns Codex gpt-5.4 ModelInfo (128k context, tools+vision)
  - healthCheck(): lightweight GET /v1/models to verify API key validity
  - createCompletion(): streaming completions via openai SDK
    chat.completions.create({ stream: true }); maps chunks to CompletionEvent
- Installs openai SDK (^6.32.0) as a dependency in apps/gateway
- Registers OpenAIAdapter in ProviderService alongside OllamaAdapter
- Removes legacy registerOpenAIProvider() private method from ProviderService

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

apps/gateway/package.json

@@ -47,6 +47,7 @@
     "dotenv": "^17.3.1",
     "fastify": "^5.0.0",
     "node-cron": "^4.2.1",
+    "openai": "^6.32.0",
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",
     "socket.io": "^4.8.0",