feat: add monitoring agent (Go) + agent report API + cloud billing UI

- Go agent: CPU/mem/disk/port collectors, HTTP report, static binary
- Cross-compile: linux/amd64 + linux/arm64 (statically linked)
- One-line install script for all Linux distros
- Backend: POST /api/v1/agent/report (no JWT) + GET /agent/latest,history (JWT)
- CloudBilling.tsx: fixed syntax errors, stats/trends/records/alerts
- K8s menu renamed to Kubernetes管理

Signed-off-by: Logan.Li

agent/collector/cpu.go

@@ -0,0 +1,122 @@
+// Package collector 提供服务器各项系统指标的采集能力。
+// 本文件实现 CPU 使用率与负载采集，主要通过解析 Linux /proc/stat 与 /proc/loadavg 实现。
+//
+// 署名 Logan.Li
+package collector
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+
+	"ezops/agent/model"
+)
+
+// cpuSample 保存某一时刻 CPU 的累计 jiffies，用于两次采样计算差值
+type cpuSample struct {
+	total uint64 // 自系统启动以来 CPU 所有状态的累计 jiffies
+	idle  uint64 // idle + iowait 的累计 jiffies
+}
+
+// CPUCollector CPU 指标采集器。
+// 通过在两次采集之间保存上一次的采样数据来计算真实使用率。
+type CPUCollector struct {
+	mu   sync.Mutex
+	prev *cpuSample
+}
+
+// NewCPUCollector 创建 CPU 采集器实例
+func NewCPUCollector() *CPUCollector {
+	return &CPUCollector{}
+}
+
+// Collect 采集当前 CPU 指标。
+// 注意：首次调用因缺少上次采样数据，返回的使用率为 0；后续调用返回两次采集之间的平均使用率。
+func (c *CPUCollector) Collect() (model.CPUStat, error) {
+	stat := model.CPUStat{CoreCount: runtime.NumCPU()}
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	cur, err := readCPUSample()
+	if err != nil {
+		return stat, fmt.Errorf("读取 CPU 采样失败: %w", err)
+	}
+
+	if c.prev != nil {
+		totalDelta := cur.total - c.prev.total
+		idleDelta := cur.idle - c.prev.idle
+		if totalDelta > 0 {
+			// 使用率 = (总增量 - 空闲增量) / 总增量 * 100
+			stat.UsagePercent = float64(totalDelta-idleDelta) / float64(totalDelta) * 100.0
+			if stat.UsagePercent < 0 {
+				stat.UsagePercent = 0
+			}
+		}
+	}
+	c.prev = &cur
+
+	// 负载平均值采集失败不影响 CPU 使用率上报
+	if l1, l5, l15, err := readLoadAvg(); err == nil {
+		stat.Load1, stat.Load5, stat.Load15 = l1, l5, l15
+	}
+
+	return stat, nil
+}
+
+// readCPUSample 读取 /proc/stat 第一行（聚合所有 CPU）并计算 total 与 idle
+func readCPUSample() (cpuSample, error) {
+	var sample cpuSample
+
+	f, err := os.Open("/proc/stat")
+	if err != nil {
+		return sample, err
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if !strings.HasPrefix(line, "cpu ") {
+			continue
+		}
+		// 形如: cpu  user nice system idle iowait irq softirq steal guest guest_nice
+		fields := strings.Fields(line)
+		if len(fields) < 5 {
+			return sample, fmt.Errorf("/proc/stat 数据格式异常")
+		}
+		// fields[0] == "cpu"，之后依次为各类 jiffies
+		for i, v := range fields[1:] {
+			x, perr := strconv.ParseUint(v, 10, 64)
+			if perr != nil {
+				continue
+			}
+			sample.total += x
+			// idle(索引3) 与 iowait(索引4) 计入空闲
+			if i == 3 || i == 4 {
+				sample.idle += x
+			}
+		}
+		return sample, nil
+	}
+	if err := scanner.Err(); err != nil {
+		return sample, err
+	}
+	return sample, fmt.Errorf("/proc/stat 中未找到聚合 CPU 行")
+}
+
+// readLoadAvg 读取 /proc/loadavg，返回 1/5/15 分钟平均负载
+func readLoadAvg() (load1, load5, load15 float64, err error) {
+	f, err := os.Open("/proc/loadavg")
+	if err != nil {
+		return 0, 0, 0, err
+	}
+	defer f.Close()
+
+	_, err = fmt.Fscanf(f, "%f %f %f", &load1, &load5, &load15)
+	return load1, load5, load15, err
+}