Initial import: nasmon health monitor, split out of home_nas

Go service (nasmon/) plus nfpm packaging (packaging/), carried over from
the home_nas monorepo. Adds rpm packaging alongside the existing deb
build (Fedora/RHEL/Rocky dependency names via nfpm overrides, per-format
systemd unit path, dpkg+rpm-compatible maintainer scripts) and a Gitea
Actions workflow that builds both formats on every push.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/collect/cpu.go

@@ -0,0 +1,75 @@
+// Package collect reads system state from /proc, /sys and CLI tools.
+package collect
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+)
+
+type CPU struct {
+	UsagePct float64    `json:"usage_pct"`
+	Cores    int        `json:"cores"`
+	Load     [3]float64 `json:"load"`
+}
+
+type cpuTimes struct{ idle, total uint64 }
+
+// CPUSampler keeps the previous /proc/stat reading to compute usage deltas.
+type CPUSampler struct {
+	prev cpuTimes
+	init bool
+}
+
+func (s *CPUSampler) Sample() (CPU, error) {
+	b, err := os.ReadFile("/proc/stat")
+	if err != nil {
+		return CPU{}, err
+	}
+	cur, err := parseProcStat(b)
+	if err != nil {
+		return CPU{}, err
+	}
+	c := CPU{Cores: runtime.NumCPU()}
+	if s.init && cur.total > s.prev.total {
+		dt := float64(cur.total - s.prev.total)
+		di := float64(cur.idle - s.prev.idle)
+		c.UsagePct = 100 * (1 - di/dt)
+	}
+	s.prev, s.init = cur, true
+	if lb, err := os.ReadFile("/proc/loadavg"); err == nil {
+		f := strings.Fields(string(lb))
+		for i := 0; i < 3 && i < len(f); i++ {
+			c.Load[i], _ = strconv.ParseFloat(f[i], 64)
+		}
+	}
+	return c, nil
+}
+
+// parseProcStat returns idle and total jiffies from the aggregate "cpu" line.
+func parseProcStat(b []byte) (cpuTimes, error) {
+	sc := bufio.NewScanner(bytes.NewReader(b))
+	for sc.Scan() {
+		f := strings.Fields(sc.Text())
+		if len(f) < 5 || f[0] != "cpu" {
+			continue
+		}
+		var t cpuTimes
+		for i, v := range f[1:] {
+			n, err := strconv.ParseUint(v, 10, 64)
+			if err != nil {
+				return t, err
+			}
+			t.total += n
+			if i == 3 || i == 4 { // idle + iowait
+				t.idle += n
+			}
+		}
+		return t, nil
+	}
+	return cpuTimes{}, fmt.Errorf("no cpu line in /proc/stat")
+}