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.

errors.go

@@ -0,0 +1,49 @@
+package bookstack
+
+import (
+	"errors"
+	"fmt"
+)
+
+// Sentinel errors for common API error conditions.
+var (
+	ErrNotFound     = errors.New("resource not found")
+	ErrUnauthorized = errors.New("unauthorized")
+	ErrForbidden    = errors.New("forbidden")
+	ErrRateLimited  = errors.New("rate limited")
+	ErrBadRequest   = errors.New("bad request")
+)
+
+// APIError represents an error returned by the Bookstack API.
+type APIError struct {
+	StatusCode int    // HTTP status code
+	Code       string // Error code from API response
+	Message    string // Error message from API response
+	Body       string // Raw response body
+}
+
+// Error implements the error interface.
+func (e *APIError) Error() string {
+	if e.Code != "" {
+		return fmt.Sprintf("bookstack API error (status %d, code %s): %s", e.StatusCode, e.Code, e.Message)
+	}
+	return fmt.Sprintf("bookstack API error (status %d): %s", e.StatusCode, e.Message)
+}
+
+// Is implements error matching for sentinel errors.
+func (e *APIError) Is(target error) bool {
+	switch target {
+	case ErrNotFound:
+		return e.StatusCode == 404
+	case ErrUnauthorized:
+		return e.StatusCode == 401
+	case ErrForbidden:
+		return e.StatusCode == 403
+	case ErrRateLimited:
+		return e.StatusCode == 429
+	case ErrBadRequest:
+		return e.StatusCode == 400
+	default:
+		return false
+	}
+}