feat(api): add system resource monitoring functionality

- implement getSystemInfo to gather CPU and memory usage
- add platform-specific implementations for memory and CPU usage
- enhance OpenAPI documentation to include system resource metrics

internal/api/system_linux.go

@@ -0,0 +1,84 @@
+//go:build linux
+
+package api
+
+import (
+	"bytes"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func getMemoryInfo() (total, used uint64) {
+	data, err := os.ReadFile("/proc/meminfo")
+	if err != nil {
+		return 0, 0
+	}
+
+	var memTotal, memAvailable uint64
+	for _, line := range strings.Split(string(data), "\n") {
+		fields := strings.Fields(line)
+		if len(fields) < 2 {
+			continue
+		}
+		val, err := strconv.ParseUint(fields[1], 10, 64)
+		if err != nil {
+			continue
+		}
+		switch fields[0] {
+		case "MemTotal:":
+			memTotal = val * 1024 // kB to bytes
+		case "MemAvailable:":
+			memAvailable = val * 1024
+		}
+	}
+
+	if memTotal > 0 && memAvailable <= memTotal {
+		return memTotal, memTotal - memAvailable
+	}
+	return memTotal, 0
+}
+
+// previous CPU times for delta calculation
+var prevIdle, prevTotal uint64
+
+func getCPUUsage() float64 {
+	data, err := os.ReadFile("/proc/stat")
+	if err != nil {
+		return 0
+	}
+
+	// first line: cpu  user nice system idle iowait irq softirq steal
+	idx := bytes.IndexByte(data, '\n')
+	if idx < 0 {
+		return 0
+	}
+	line := string(data[:idx])
+	fields := strings.Fields(line)
+	if len(fields) < 5 || fields[0] != "cpu" {
+		return 0
+	}
+
+	var total, idle uint64
+	for i := 1; i < len(fields); i++ {
+		val, err := strconv.ParseUint(fields[i], 10, 64)
+		if err != nil {
+			continue
+		}
+		total += val
+		if i == 4 { // idle is the 4th value (index 4 in fields, 1-based field 4)
+			idle = val
+		}
+	}
+
+	deltaTotal := total - prevTotal
+	deltaIdle := idle - prevIdle
+	prevIdle = idle
+	prevTotal = total
+
+	if deltaTotal == 0 {
+		return 0
+	}
+
+	return float64(deltaTotal-deltaIdle) / float64(deltaTotal) * 100
+}