Set up Go module and project structure

- Populated .gitignore with Go-specific patterns
- Created placeholder Go files with basic package structure:
  - bookstack.go: Client and Config setup
  - types.go: Data structures (Book, Page, Chapter, Shelf, SearchResult)
  - errors.go: Error handling types with sentinel errors
  - http.go: HTTP helper placeholder with ListOptions
  - books.go, pages.go, chapters.go, shelves.go, search.go: Service placeholders
- Verified build succeeds with go build ./...
- No external dependencies added (stdlib only)

All files compile successfully and follow flat package structure.

shelves.go

@@ -0,0 +1,43 @@
+package bookstack
+
+import "context"
+
+// ShelvesService handles operations on shelves.
+type ShelvesService struct {
+	client *Client
+}
+
+// List returns a list of shelves with optional filtering.
+// TODO: Implement API call to GET /api/shelves
+func (s *ShelvesService) List(ctx context.Context, opts *ListOptions) ([]Shelf, error) {
+	// Placeholder for future implementation
+	return nil, nil
+}
+
+// Get retrieves a single shelf by ID.
+// TODO: Implement API call to GET /api/shelves/{id}
+func (s *ShelvesService) Get(ctx context.Context, id int) (*Shelf, error) {
+	// Placeholder for future implementation
+	return nil, nil
+}
+
+// Create creates a new shelf.
+// TODO: Implement API call to POST /api/shelves
+func (s *ShelvesService) Create(ctx context.Context, shelf *Shelf) (*Shelf, error) {
+	// Placeholder for future implementation
+	return nil, nil
+}
+
+// Update updates an existing shelf.
+// TODO: Implement API call to PUT /api/shelves/{id}
+func (s *ShelvesService) Update(ctx context.Context, id int, shelf *Shelf) (*Shelf, error) {
+	// Placeholder for future implementation
+	return nil, nil
+}
+
+// Delete deletes a shelf by ID.
+// TODO: Implement API call to DELETE /api/shelves/{id}
+func (s *ShelvesService) Delete(ctx context.Context, id int) error {
+	// Placeholder for future implementation
+	return nil
+}