refactor: consolidate AgentFile struct into common package

Created aggregator/pkg/common module with shared AgentFile type.
Removed duplicate definitions from migration and services packages.
Both agent and server now use common.AgentFile.

aggregator/go.mod

@@ -0,0 +1,3 @@
+module github.com/Fimeg/RedFlag/aggregator
+
+go 1.23.0

aggregator/pkg/common/agentfile.go

@@ -0,0 +1,44 @@
+package common
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"os"
+	"time"
+)
+
+type AgentFile struct {
+	Path         string    `json:"path"`
+	Size         int64     `json:"size"`
+	ModifiedTime time.Time `json:"modified_time"`
+	Version      string    `json:"version,omitempty"`
+	Checksum     string    `json:"checksum"`
+	Required     bool      `json:"required"`
+	Migrate      bool      `json:"migrate"`
+	Description  string    `json:"description"`
+}
+
+// CalculateChecksum computes SHA256 checksum of a file
+func CalculateChecksum(filePath string) (string, error) {
+	data, err := os.ReadFile(filePath)
+	if err != nil {
+		return "", err
+	}
+	hash := sha256.Sum256(data)
+	return hex.EncodeToString(hash[:]), nil
+}
+
+// IsRequiredFile determines if a file is required for agent operation
+func IsRequiredFile(path string) bool {
+	requiredFiles := []string{
+		"/etc/redflag/config.json",
+		"/usr/local/bin/redflag-agent",
+		"/etc/systemd/system/redflag-agent.service",
+	}
+	for _, rf := range requiredFiles {
+		if path == rf {
+			return true
+		}
+	}
+	return false
+}