fix: 全项目文档对齐 + 代码清理

代码修复:
- web/agent/agent.py: datetime.utcnow() → datetime.now(timezone.utc)
- requirements.txt: 移除 paramiko==3.5.0 (已无活跃引用)
- 删除 server/infrastructure/ssh/pool.py (DEPRECATED, 无引用)

连接池三层对齐 (config.py/.env.example/session.py):
- DB_POOL_SIZE 100→160, DB_MAX_OVERFLOW 100→120
- 基于 MySQL max_connections=400, install.php 公式

文档修复 (21项):
- P0: 硬编码域名/IP替换为配置变量占位符
- P1: 10个过时设计文档加归档标注 (引用旧文件结构)
- P2: step-3-webssh报告 paramiko引用修正
- 6份审查报告连接池参数勘误 100/100→160/120
- ECC安全报告 EC5 datetime.utcnow 标记已修复
- docs/README.md 文档索引重写
- docs/memory/mem_nexus_overview.md 移除硬编码凭证
- docs/project/tech-stack-inventory.md paramiko标记已移除

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

server/application/services/server_service.py

@@ -9,7 +9,6 @@ from typing import Optional, List, Dict
 
 from server.domain.models import Server, SyncLog
 from server.domain.repositories import ServerRepository, SyncLogRepository
-from server.infrastructure.ssh.pool import SSHConfig, exec_command
 from server.infrastructure.redis.client import get_redis
 
 logger = logging.getLogger("nexus.server_service")