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/fs.go

@@ -0,0 +1,20 @@
+//go:build linux
+
+package collect
+
+import "syscall"
+
+type FSUsage struct {
+	Path  string `json:"path"`
+	Total uint64 `json:"total"`
+	Free  uint64 `json:"free"`
+}
+
+func ReadFS(path string) (FSUsage, error) {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(path, &st); err != nil {
+		return FSUsage{}, err
+	}
+	bs := uint64(st.Bsize)
+	return FSUsage{Path: path, Total: st.Blocks * bs, Free: st.Bavail * bs}, nil
+}