feat(bookstack-api-02m): create example code in examples/ directory

Add three example programs: basic (list books, get page), search
(full-text search with CLI args), and export (markdown/PDF export).

examples/export/main.go

@@ -0,0 +1,59 @@
+// Export example: export a page to markdown or PDF.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+
+	bookstack "code.beautifulmachines.dev/jakoubek/bookstack-api"
+)
+
+func main() {
+	if len(os.Args) < 3 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <page-id> <markdown|pdf>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	pageID, err := strconv.Atoi(os.Args[1])
+	if err != nil {
+		log.Fatalf("Invalid page ID: %s", os.Args[1])
+	}
+	format := os.Args[2]
+
+	client, err := bookstack.NewClient(bookstack.Config{
+		BaseURL:     os.Getenv("BOOKSTACK_URL"),
+		TokenID:     os.Getenv("BOOKSTACK_TOKEN_ID"),
+		TokenSecret: os.Getenv("BOOKSTACK_TOKEN_SECRET"),
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	ctx := context.Background()
+
+	switch format {
+	case "markdown":
+		data, err := client.Pages.ExportMarkdown(ctx, pageID)
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Print(string(data))
+
+	case "pdf":
+		data, err := client.Pages.ExportPDF(ctx, pageID)
+		if err != nil {
+			log.Fatal(err)
+		}
+		filename := fmt.Sprintf("page-%d.pdf", pageID)
+		if err := os.WriteFile(filename, data, 0644); err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("Saved to %s (%d bytes)\n", filename, len(data))
+
+	default:
+		log.Fatalf("Unknown format: %s (use markdown or pdf)", format)
+	}
+}