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).

src/agent-runner.ts

@@ -214,7 +214,7 @@ function prepareGroupEnvironment(
     const authSrc = path.join(hostCodexDir, 'auth.json');
     const authDst = path.join(sessionCodexDir, 'auth.json');
     if (fs.existsSync(authSrc)) fs.copyFileSync(authSrc, authDst);
-    for (const file of ['config.toml', 'config.json', 'instructions.md']) {
+    for (const file of ['config.toml', 'config.json', 'AGENTS.md']) {
       const src = path.join(hostCodexDir, file);
       const dst = path.join(sessionCodexDir, file);
       if (fs.existsSync(src)) {