fix(deploy): SSH 연결 재시도 로직 추가 (kex_exchange 거부 대응)

- 본 명령 실행 전 SSH 연결 테스트 (최대 3회, 10초 간격)
- ServerAliveInterval=15 추가

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

.gitea/workflows/deploy.yml

@@ -92,7 +92,21 @@ jobs:
           chmod 600 ~/.ssh/id_deploy
           ssh-keyscan -T 5 $DEPLOY_HOST >> ~/.ssh/known_hosts 2>/dev/null || true
 
-          SSH_CMD="ssh -i ~/.ssh/id_deploy -o StrictHostKeyChecking=no -o ConnectTimeout=10 root@$DEPLOY_HOST"
+          SSH_CMD="ssh -i ~/.ssh/id_deploy -o StrictHostKeyChecking=no -o ConnectTimeout=10 -o ServerAliveInterval=15 root@$DEPLOY_HOST"
+
+          # SSH 연결 테스트 (최대 3회, kex_exchange 거부 대응)
+          for attempt in 1 2 3; do
+            echo "SSH connectivity test $attempt/3..."
+            if $SSH_CMD echo "SSH OK"; then
+              break
+            fi
+            if [ "$attempt" -eq 3 ]; then
+              echo "ERROR: SSH connection failed after 3 attempts"
+              exit 1
+            fi
+            echo "SSH failed, retrying in 10s..."
+            sleep 10
+          done
 
           $SSH_CMD bash -s << 'RESTART'
           DEPLOY_DIR=/devdata/services/kcg/backend