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_darwin.go

@@ -0,0 +1,104 @@
+//go:build darwin
+
+package api
+
+import (
+	"os/exec"
+	"runtime"
+	"strconv"
+	"strings"
+	"syscall"
+	"unsafe"
+)
+
+func getMemoryInfo() (total, used uint64) {
+	total = sysctl64("hw.memsize")
+	if total == 0 {
+		return 0, 0
+	}
+
+	pageSize, err := syscall.SysctlUint32("hw.pagesize")
+	if err != nil {
+		return total, 0
+	}
+
+	freeCount, _ := syscall.SysctlUint32("vm.page_free_count")
+	purgeableCount, _ := syscall.SysctlUint32("vm.page_purgeable_count")
+	speculativeCount, _ := syscall.SysctlUint32("vm.page_speculative_count")
+
+	// inactive pages not available via sysctl, parse vm_stat
+	inactiveCount := vmStatPages("Pages inactive")
+
+	available := uint64(freeCount+purgeableCount+speculativeCount)*uint64(pageSize) +
+		inactiveCount*uint64(pageSize)
+	if available > total {
+		return total, 0
+	}
+	return total, total - available
+}
+
+// vmStatPages parses vm_stat output for a specific counter
+func vmStatPages(key string) uint64 {
+	out, err := exec.Command("vm_stat").Output()
+	if err != nil {
+		return 0
+	}
+	for _, line := range strings.Split(string(out), "\n") {
+		if strings.HasPrefix(line, key) {
+			// format: "Pages inactive:          479321."
+			parts := strings.Split(line, ":")
+			if len(parts) < 2 {
+				continue
+			}
+			s := strings.TrimSpace(parts[1])
+			s = strings.TrimSuffix(s, ".")
+			val, err := strconv.ParseUint(s, 10, 64)
+			if err != nil {
+				return 0
+			}
+			return val
+		}
+	}
+	return 0
+}
+
+func sysctl64(name string) uint64 {
+	s, err := syscall.Sysctl(name)
+	if err != nil {
+		return 0
+	}
+	b := []byte(s)
+	for len(b) < 8 {
+		b = append(b, 0)
+	}
+	return *(*uint64)(unsafe.Pointer(&b[0]))
+}
+
+func getCPUUsage() float64 {
+	s, err := syscall.Sysctl("vm.loadavg")
+	if err != nil {
+		return 0
+	}
+
+	raw := []byte(s)
+	for len(raw) < 24 {
+		raw = append(raw, 0)
+	}
+
+	// struct loadavg { fixpt_t ldavg[3]; long fscale; }
+	ldavg0 := *(*uint32)(unsafe.Pointer(&raw[0]))
+	fscale := *(*int64)(unsafe.Pointer(&raw[16]))
+
+	if fscale == 0 {
+		return 0
+	}
+
+	load1 := float64(ldavg0) / float64(fscale)
+	numCPU := float64(runtime.NumCPU())
+
+	usage := load1 / numCPU * 100
+	if usage > 100 {
+		usage = 100
+	}
+	return usage
+}