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/root.go

@@ -34,6 +34,10 @@ with full PGP encryption support.`,
 	cmd.AddCommand(attachmentCmd())
 	cmd.AddCommand(folderCmd())
 	cmd.AddCommand(labelCmd())
+	cmd.AddCommand(accountsCmd())
+	cmd.AddCommand(webhookCmd())
+	cmd.AddCommand(pgpCmd())
+	cmd.AddCommand(pluginCmd())
 	return cmd
 }
 
@@ -47,6 +51,10 @@ func NewRootCmd() *cobra.Command {
 	rootCmd.AddCommand(attachmentCmd())
 	rootCmd.AddCommand(folderCmd())
 	rootCmd.AddCommand(labelCmd())
+	rootCmd.AddCommand(accountsCmd())
+	rootCmd.AddCommand(webhookCmd())
+	rootCmd.AddCommand(pgpCmd())
+	rootCmd.AddCommand(pluginCmd())
 	return rootCmd
 }