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/plugin.go

@@ -0,0 +1,111 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/frenocorp/pop/internal/plugin"
+	"github.com/spf13/cobra"
+)
+
+func pluginCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "plugin",
+		Short: "Manage plugins",
+		Long:  `List, enable, disable, and manage plugins for the Pop CLI.`,
+	}
+
+	cmd.AddCommand(pluginListCmd())
+	cmd.AddCommand(pluginEnableCmd())
+	cmd.AddCommand(pluginDisableCmd())
+
+	return cmd
+}
+
+func pluginListCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "list",
+		Short: "List all registered plugins",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			reg, err := plugin.NewPluginRegistry()
+			if err != nil {
+				return fmt.Errorf("failed to create plugin registry: %w", err)
+			}
+
+			plugins, err := reg.ListPlugins()
+			if err != nil {
+				return fmt.Errorf("failed to list plugins: %w", err)
+			}
+
+			if len(plugins) == 0 {
+				fmt.Println("No plugins registered.")
+				return nil
+			}
+
+			for _, p := range plugins {
+				fmt.Printf("Name: %s\n  Version: %s\n  Description: %s\n  Binary: %s\n\n",
+					p.Name, p.Version, p.Description, p.Binary)
+			}
+
+			return nil
+		},
+	}
+}
+
+func pluginEnableCmd() *cobra.Command {
+	var name string
+
+	cmd := &cobra.Command{
+		Use:   "enable <name>",
+		Short: "Enable a plugin",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			name = args[0]
+
+			reg, err := plugin.NewPluginRegistry()
+			if err != nil {
+				return fmt.Errorf("failed to create plugin registry: %w", err)
+			}
+
+			if err := reg.EnablePlugin(name); err != nil {
+				return fmt.Errorf("failed to enable plugin: %w", err)
+			}
+
+			fmt.Printf("Enabled plugin: %s\n", name)
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+func pluginDisableCmd() *cobra.Command {
+	var name string
+
+	cmd := &cobra.Command{
+		Use:   "disable <name>",
+		Short: "Disable a plugin",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			name = args[0]
+
+			reg, err := plugin.NewPluginRegistry()
+			if err != nil {
+				return fmt.Errorf("failed to create plugin registry: %w", err)
+			}
+
+			if err := reg.DisablePlugin(name); err != nil {
+				return fmt.Errorf("failed to disable plugin: %w", err)
+			}
+
+			fmt.Printf("Disabled plugin: %s\n", name)
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+func init() {
+	_ = os.Getenv
+}