FRE-683: Add contacts & attachments management

- Implemented contact CRUD operations (list, add, edit, delete)
- Implemented attachment management (list, upload, download)
- Created internal/contact/manager.go for contact persistence
- Created internal/attachment/manager.go for attachment storage
- Added CLI commands in cmd/contacts.go and cmd/attachments.go
- Integrated contact and attachment commands into root CLI

Files:
- internal/contact/types.go - Contact data models
- internal/contact/manager.go - Contact CRUD operations
- internal/attachment/manager.go - Attachment file operations
- cmd/contacts.go - Contact CLI commands
- cmd/attachments.go - Attachment CLI commands

Contacts stored in ~/.config/pop/contacts.json
Attachments stored in ~/.config/pop/attachments/

cmd/attachments.go

@@ -0,0 +1,124 @@
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/frenocorp/pop/internal/attachment"
+	"github.com/spf13/cobra"
+)
+
+func attachmentCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "attachment",
+		Short: "Manage attachments",
+		Long:  `List, download, and upload attachments.`,
+	}
+
+	cmd.AddCommand(attachmentListCmd())
+	cmd.AddCommand(attachmentDownloadCmd())
+	cmd.AddCommand(attachmentUploadCmd())
+
+	return cmd
+}
+
+func attachmentListCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "list",
+		Short: "List all attachments",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			manager := attachment.NewAttachmentManager()
+			
+			ids, err := manager.List()
+			if err != nil {
+				return fmt.Errorf("failed to list attachments: %w", err)
+			}
+			
+			if len(ids) == 0 {
+				fmt.Println("No attachments found")
+				return nil
+			}
+			
+			fmt.Println("Attachments:")
+			for _, id := range ids {
+				fmt.Printf("  - %s\n", id)
+			}
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+func attachmentDownloadCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "download <attachment-id> [output-dir]",
+		Short: "Download an attachment",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if len(args) < 1 {
+				return fmt.Errorf("attachment ID is required")
+			}
+			
+			id := args[0]
+			outputDir := "."
+			if len(args) > 1 {
+				outputDir = args[1]
+			}
+			
+			manager := attachment.NewAttachmentManager()
+			
+			data, err := manager.Get(id)
+			if err != nil {
+				return fmt.Errorf("failed to get attachment: %w", err)
+			}
+			
+			if err := manager.Download(id, id, outputDir); err != nil {
+				return fmt.Errorf("failed to download attachment: %w", err)
+			}
+			
+			fmt.Printf("Downloaded attachment %s to %s\n", id, outputDir)
+			fmt.Printf("Size: %d bytes\n", len(data))
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+func attachmentUploadCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "upload <attachment-id> <file-path>",
+		Short: "Upload an attachment",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if len(args) < 2 {
+				return fmt.Errorf("attachment ID and file path are required")
+			}
+			
+			id := args[0]
+			filePath := args[1]
+			
+			file, err := os.Open(filePath)
+			if err != nil {
+				return fmt.Errorf("failed to open file: %w", err)
+			}
+			defer file.Close()
+			
+			manager := attachment.NewAttachmentManager()
+			
+			if err := manager.Upload(id, file.Name(), file); err != nil {
+				return fmt.Errorf("failed to upload attachment: %w", err)
+			}
+			
+			info, _ := file.Stat()
+			fmt.Printf("Uploaded attachment %s (%s, %d bytes)\n", id, file.Name(), info.Size())
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+func readAll(r io.Reader) ([]byte, error) {
+	return io.ReadAll(r)
+}