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>

pnpm-lock.yaml

@@ -143,6 +143,9 @@ importers:
       node-cron:
         specifier: ^4.2.1
         version: 4.2.1
+      openai:
+        specifier: ^6.32.0
+        version: 6.32.0(ws@8.19.0)(zod@4.3.6)
       reflect-metadata:
         specifier: ^0.2.0
         version: 0.2.2
@@ -4879,6 +4882,18 @@ packages:
       zod:
         optional: true
 
+  openai@6.32.0:
+    resolution: {integrity: sha512-j3k+BjydAf8yQlcOI7WUQMQTbbF5GEIMAE2iZYCOzwwB3S2pCheaWYp+XZRNAch4jWVc52PMDGRRjutao3lLCg==}
+    hasBin: true
+    peerDependencies:
+      ws: ^8.18.0
+      zod: ^3.25 || ^4.0
+    peerDependenciesMeta:
+      ws:
+        optional: true
+      zod:
+        optional: true
+
   optionator@0.9.4:
     resolution: {integrity: sha512-6IpQ7mKUxRcZNLIObR0hz7lxsapSSIYNZJwXPGeF0mTVqGKFIXj1DQcMoT22S3ROcLyY/rz0PWaWZ9ayWmad9g==}
     engines: {node: '>= 0.8.0'}
@@ -10621,6 +10636,11 @@ snapshots:
       ws: 8.19.0
       zod: 4.3.6
 
+  openai@6.32.0(ws@8.19.0)(zod@4.3.6):
+    optionalDependencies:
+      ws: 8.19.0
+      zod: 4.3.6
+
   optionator@0.9.4:
     dependencies:
       deep-is: 0.1.4