FRE-680: Initial project scaffold with auth & API client

- Set up Go module with Cobra CLI skeleton
- Implemented login/logout/session commands with 2FA support
- Created ProtonMail API client with rate limiting
- Added config management for ~/.config/pop/
- Configured CI/CD pipeline with GitHub Actions
- Added Makefile for build/test/lint targets

Files:
- main.go, go.mod, go.sum
- cmd/root.go, cmd/auth.go
- internal/auth/session.go
- internal/config/config.go
- internal/api/client.go
- Makefile, README.md, .gitignore
- .github/workflows/ci.yml

cmd/root.go

@@ -0,0 +1,41 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var rootCmd = &cobra.Command{
+	Use:   "pop",
+	Short: "ProtonMail CLI tool",
+	Long: `pop is a CLI tool for interacting with ProtonMail.
+
+It provides commands for managing emails, contacts, and attachments
+with full PGP encryption support.`,
+}
+
+func NewRootCmd() *cobra.Command {
+	rootCmd.AddCommand(loginCmd())
+	rootCmd.AddCommand(logoutCmd())
+	rootCmd.AddCommand(sessionCmd())
+
+	return rootCmd
+}
+
+func init() {
+	cobra.OnInitialize(initConfig)
+}
+
+func initConfig() {
+	// Initialize config loading
+	// Config is loaded from ~/.config/pop/
+}
+
+func Execute() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
+	}
+}