refactor: switch codex runner from app-server to SDK

Replace raw JSON-RPC app-server protocol with @openai/codex-sdk.
The SDK wraps `codex exec` which ensures complete task execution per
turn, fixing the issue where app-server mode ended turns prematurely
(agent saying "하겠습니다" without doing the work).

Also fix AGENTS.md copy in agent-runner (was copying instructions.md
which Codex CLI doesn't read).

container/codex-runner/package.json

@@ -8,7 +8,9 @@
     "build": "tsc",
     "start": "node dist/index.js"
   },
-  "dependencies": {},
+  "dependencies": {
+    "@openai/codex-sdk": "^0.114.0"
+  },
   "devDependencies": {
     "@types/node": "^22.10.7",
     "typescript": "^5.7.3"