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/auth.go

@@ -0,0 +1,60 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/frenocorp/pop/internal/auth"
+	"github.com/spf13/cobra"
+)
+
+func loginCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "login",
+		Short: "Log in to ProtonMail",
+		Long:  `Authenticate with ProtonMail API and store session credentials.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			manager := auth.NewSessionManager()
+			return manager.Login()
+		},
+	}
+
+	cmd.Flags().StringP("email", "e", "", "ProtonMail email address")
+	cmd.Flags().StringP("password", "p", "", "ProtonMail password")
+	cmd.Flags().BoolP("interactive", "i", true, "Interactive prompt for credentials")
+
+	return cmd
+}
+
+func logoutCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "logout",
+		Short: "Log out from ProtonMail",
+		Long:  `Clear stored session credentials.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			manager := auth.NewSessionManager()
+			return manager.Logout()
+		},
+	}
+
+	return cmd
+}
+
+func sessionCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "session",
+		Short: "Show current session info",
+		Long:  `Display current authentication session details.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			manager := auth.NewSessionManager()
+			session, err := manager.GetSession()
+			if err != nil {
+				return fmt.Errorf("no active session: %w", err)
+			}
+			fmt.Fprintf(os.Stdout, "Session: %s\n", session.UID)
+			return nil
+		},
+	}
+
+	return cmd
+}