feat: implement Milestone 3 integration points

Add comprehensive integration capabilities to Pop CLI:

- Multi-account support with named profiles
- Webhook management with signature verification
- External PGP key management (import/export/encrypt/decrypt/sign/verify)
- CLI plugin system for extensibility
- Complete documentation in README.md

All compilation errors fixed and build verified CLEAN.

Security review delegated to FRE-5202.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

cmd/accounts.go

@@ -0,0 +1,210 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"text/tabwriter"
+
+	"github.com/frenocorp/pop/internal/accounts"
+	"github.com/spf13/cobra"
+)
+
+func accountsCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "accounts",
+		Short: "Manage multiple ProtonMail accounts",
+		Long:  `Add, list, switch, and remove named ProtonMail account profiles.`,
+	}
+
+	cmd.AddCommand(accountsListCmd())
+	cmd.AddCommand(accountsAddCmd())
+	cmd.AddCommand(accountsRemoveCmd())
+	cmd.AddCommand(accountsDefaultCmd())
+	cmd.AddCommand(accountsShowCmd())
+
+	return cmd
+}
+
+func accountsListCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "list",
+		Short: "List all configured accounts",
+		Long:  `Show all saved ProtonMail account profiles.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			store, err := accounts.NewAccountsStore()
+			if err != nil {
+				return fmt.Errorf("failed to create accounts store: %w", err)
+			}
+
+			accts, err := store.LoadAccounts()
+			if err != nil {
+				return fmt.Errorf("failed to load accounts: %w", err)
+			}
+
+			if len(accts) == 0 {
+				fmt.Println("No accounts configured. Use 'pop accounts add' to create one.")
+				return nil
+			}
+
+			w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
+			fmt.Fprintln(w, "Name\tEmail\tDefault\tAPI Base\tCreated")
+			fmt.Fprintln(w, "----\t-----\t-------\t--------\t-------")
+
+			for _, acc := range accts {
+				def := "-"
+				if acc.Default {
+					def = "*"
+				}
+				fmt.Fprintf(w, "%s\t%s\t%s\t%s\t%s\n",
+					acc.Name, acc.Email, def, acc.APIBaseURL, acc.CreatedAt)
+			}
+
+			return w.Flush()
+		},
+	}
+}
+
+func accountsAddCmd() *cobra.Command {
+	var name, email, apiURL string
+	var isDefault bool
+
+	cmd := &cobra.Command{
+		Use:   "add <name>",
+		Short: "Add a new account profile",
+		Long:  `Add a named ProtonMail account profile for multi-account support.`,
+		Args:  cobra.MaximumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			accountName := name
+			if len(args) > 0 && args[0] != "" {
+				accountName = args[0]
+			}
+			if accountName == "" {
+				return fmt.Errorf("account name is required")
+			}
+			if email == "" {
+				return fmt.Errorf("email is required (--email)")
+			}
+
+			store, err := accounts.NewAccountsStore()
+			if err != nil {
+				return fmt.Errorf("failed to create accounts store: %w", err)
+			}
+
+			if err := store.AddAccount(accountName, email, apiURL, isDefault); err != nil {
+				return fmt.Errorf("failed to add account: %w", err)
+			}
+
+			fmt.Printf("Added account: %s (%s)\n", accountName, email)
+			if isDefault {
+				fmt.Println("Set as default account")
+			}
+			return nil
+		},
+	}
+
+	cmd.Flags().StringVar(&name, "name", "", "Account name (or pass as positional argument)")
+	cmd.Flags().StringVarP(&email, "email", "e", "", "ProtonMail email address")
+	cmd.Flags().StringVar(&apiURL, "api-url", "", "Custom API base URL")
+	cmd.Flags().BoolVarP(&isDefault, "default", "d", false, "Set as default account")
+	_ = cmd.MarkFlagRequired("email")
+
+	return cmd
+}
+
+func accountsRemoveCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "remove <name>",
+		Short: "Remove an account profile",
+		Long:  `Remove a named ProtonMail account profile.`,
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			accountName := args[0]
+
+			store, err := accounts.NewAccountsStore()
+			if err != nil {
+				return fmt.Errorf("failed to create accounts store: %w", err)
+			}
+
+			if err := store.RemoveAccount(accountName); err != nil {
+				return fmt.Errorf("failed to remove account: %w", err)
+			}
+
+			fmt.Printf("Removed account: %s\n", accountName)
+			return nil
+		},
+	}
+}
+
+func accountsDefaultCmd() *cobra.Command {
+	var setName string
+
+	cmd := &cobra.Command{
+		Use:   "default [name]",
+		Short: "Get or set the default account",
+		Long:  `Show the current default account, or set a new default.`,
+		Args:  cobra.MaximumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			store, err := accounts.NewAccountsStore()
+			if err != nil {
+				return fmt.Errorf("failed to create accounts store: %w", err)
+			}
+
+			if setName == "" && len(args) > 0 {
+				setName = args[0]
+			}
+
+			if setName != "" {
+				if err := store.SetDefaultAccount(setName); err != nil {
+					return fmt.Errorf("failed to set default account: %w", err)
+				}
+				fmt.Printf("Set default account to: %s\n", setName)
+				return nil
+			}
+
+			acct, err := store.GetAccount("")
+			if err != nil {
+				return fmt.Errorf("no default account: %w", err)
+			}
+			fmt.Printf("Default account: %s (%s)\n", acct.Name, acct.Email)
+			return nil
+		},
+	}
+
+	cmd.Flags().StringVar(&setName, "set", "", "Set default account to this name")
+
+	return cmd
+}
+
+func accountsShowCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "show <name>",
+		Short: "Show account details",
+		Long:  `Display details for a specific account profile.`,
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			accountName := args[0]
+
+			store, err := accounts.NewAccountsStore()
+			if err != nil {
+				return fmt.Errorf("failed to create accounts store: %w", err)
+			}
+
+			acct, err := store.GetAccount(accountName)
+			if err != nil {
+				return fmt.Errorf("failed to get account: %w", err)
+			}
+
+			fmt.Printf("Name:       %s\n", acct.Name)
+			fmt.Printf("Email:      %s\n", acct.Email)
+			fmt.Printf("UID:        %s\n", acct.UID)
+			fmt.Printf("API Base:   %s\n", acct.APIBaseURL)
+			fmt.Printf("Default:    %t\n", acct.Default)
+			fmt.Printf("Created:    %s\n", acct.CreatedAt)
+			if acct.LastUsedAt != "" {
+				fmt.Printf("Last Used:  %s\n", acct.LastUsedAt)
+			}
+
+			return nil
+		},
+	}
+}