feat: add system stats API for dashboard monitoring

- GET /tools/system/stats - Real-time host system statistics
- CPU usage, memory, all mounted disks, network I/O
- GPU/VRAM stats via nvidia-smi (if available)
- Uses psutil for cross-platform host metrics
- Auto-discovers and filters real filesystems

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/domains/tools/__init__.py

@@ -1,9 +1,16 @@
 """
 Tools Domain
 
-Provides utility tool endpoints including DNS lookups.
+Provides utility tool endpoints including DNS lookups and system stats.
 """
 from src.domains.tools.controller import tools_controller
 from src.domains.tools.dns import DNSService, DNSQueryError
+from src.domains.tools.system import SystemStatsService, SystemStatsResponse
 
-__all__ = ["tools_controller", "DNSService", "DNSQueryError"]
+__all__ = [
+    "tools_controller",
+    "DNSService",
+    "DNSQueryError",
+    "SystemStatsService",
+    "SystemStatsResponse",
+]