feat: add project export and import functionality

- Implemented `pcli project export` command to export project hierarchy as JSON.
- Added `pcli project import` command to import project data from JSON.
- Created user client to fetch user details for comment attribution.
- Introduced new data structures for export and import processes.
- Ensured name-based references in exports and handled conflicts during imports.
- Added versioning and progress reporting for import operations.
- Updated documentation and specifications for new features.

cmd/export.go

@@ -0,0 +1,42 @@
+package cmd
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var projectExportCmd = &cobra.Command{
+	Use:   "export <project-id-or-name>",
+	Short: "Export a project to JSON",
+	Long:  "Export a project (or a specific board) as a portable JSON file to stdout",
+	Args:  cobra.ExactArgs(1),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		boardFilter, _ := cmd.Flags().GetString("board")
+
+		projectID, err := resolveProject(args[0])
+		if err != nil {
+			return err
+		}
+
+		envelope, err := getClient().ExportProject(getContext(), projectID, boardFilter)
+		if err != nil {
+			return friendlyAPIError(err, "export project", "")
+		}
+
+		encoder := json.NewEncoder(os.Stdout)
+		encoder.SetIndent("", "  ")
+		if err := encoder.Encode(envelope); err != nil {
+			return fmt.Errorf("failed to encode export: %w", err)
+		}
+
+		return nil
+	},
+}
+
+func init() {
+	projectCmd.AddCommand(projectExportCmd)
+	projectExportCmd.Flags().String("board", "", "Filter to a specific board (name or ID)")
+}