refactor(edge): 抽取边缘运行时共享包并完成 Phase 3 重构

- 新增 internal/apps/edge/，三组件改为薄包装，删除 3000+ 行重复代码
- Agent 心跳周期下沉至 heartbeat/cycle.go
- 协议类型迁入 pkg/protocol/agent.go
- 补充设计文档与 changelog

internal/apps/edge/observability/linux_test.go

@@ -0,0 +1,78 @@
+package observability
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestParseMemInfoValue(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		line string
+		want int64
+	}{
+		{line: "MemTotal:       16384000 kB", want: 16384000},
+		{line: "MemAvailable:   8192000 kB", want: 8192000},
+		{line: "invalid", want: 0},
+		{line: "MemTotal: not-a-number kB", want: 0},
+	}
+
+	for _, tt := range tests {
+		if got := parseMemInfoValue(tt.line); got != tt.want {
+			t.Fatalf("parseMemInfoValue(%q) = %d, want %d", tt.line, got, tt.want)
+		}
+	}
+}
+
+func TestShouldSkipDiskDevice(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		device string
+		want   bool
+	}{
+		{device: "", want: true},
+		{device: "loop0", want: true},
+		{device: "ram0", want: true},
+		{device: "dm-0", want: true},
+		{device: "sda", want: false},
+		{device: "nvme0n1", want: false},
+	}
+
+	for _, tt := range tests {
+		if got := shouldSkipDiskDevice(tt.device); got != tt.want {
+			t.Fatalf("shouldSkipDiskDevice(%q) = %v, want %v", tt.device, got, tt.want)
+		}
+	}
+}
+
+func TestReadFirstLine(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sample.txt")
+	if err := os.WriteFile(path, []byte("  first line\nsecond line\n"), 0o644); err != nil {
+		t.Fatalf("write file: %v", err)
+	}
+
+	if got := ReadFirstLine(path); got != "first line\nsecond line" {
+		t.Fatalf("ReadFirstLine() = %q, want trimmed first line content", got)
+	}
+	if got := ReadFirstLine(filepath.Join(dir, "missing.txt")); got != "" {
+		t.Fatalf("ReadFirstLine(missing) = %q, want empty string", got)
+	}
+}
+
+func TestStatFilesystem(t *testing.T) {
+	t.Parallel()
+
+	total, used := StatFilesystem(t.TempDir())
+	if total <= 0 {
+		t.Fatalf("StatFilesystem() total = %d, want > 0", total)
+	}
+	if used < 0 || used > total {
+		t.Fatalf("StatFilesystem() used = %d, total = %d", used, total)
+	}
+}