Introduce structured error types and bump Go/linter versions

Replace ad-hoc fmt.Errorf/errors.New calls with typed error structs across config, sarin, and script packages to enable type-based error handling. Add script-specific error handlers in CLI entry point. Fix variable shadowing bug in Worker for scriptTransformer. Bump Go to 1.25.7 and golangci-lint to v2.8.0.

internal/sarin/filecache.go

@@ -1,7 +1,6 @@
 package sarin
 
 import (
-	"fmt"
 	"io"
 	"net/http"
 	"os"
@@ -10,6 +9,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"go.aykhans.me/sarin/internal/types"
 )
 
 // CachedFile holds the cached content and metadata of a file.
@@ -31,6 +32,10 @@ func NewFileCache(requestTimeout time.Duration) *FileCache {
 
 // GetOrLoad retrieves a file from cache or loads it using the provided source.
 // The source can be a local file path or an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.FileReadError
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func (fc *FileCache) GetOrLoad(source string) (*CachedFile, error) {
 	if val, ok := fc.cache.Load(source); ok {
 		return val.(*CachedFile), nil
@@ -59,14 +64,21 @@ func (fc *FileCache) GetOrLoad(source string) (*CachedFile, error) {
 	return actual.(*CachedFile), nil
 }
 
+// readLocalFile reads a file from the local filesystem and returns its content and filename.
+// It can return the following errors:
+//   - types.FileReadError
 func (fc *FileCache) readLocalFile(filePath string) ([]byte, string, error) {
 	content, err := os.ReadFile(filePath) //nolint:gosec
 	if err != nil {
-		return nil, "", fmt.Errorf("failed to read file %s: %w", filePath, err)
+		return nil, "", types.NewFileReadError(filePath, err)
 	}
 	return content, filepath.Base(filePath), nil
 }
 
+// fetchURL downloads file contents from an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func (fc *FileCache) fetchURL(url string) ([]byte, string, error) {
 	client := &http.Client{
 		Timeout: fc.requestTimeout,
@@ -74,17 +86,17 @@ func (fc *FileCache) fetchURL(url string) ([]byte, string, error) {
 
 	resp, err := client.Get(url)
 	if err != nil {
-		return nil, "", fmt.Errorf("failed to fetch URL %s: %w", url, err)
+		return nil, "", types.NewHTTPFetchError(url, err)
 	}
 	defer resp.Body.Close() //nolint:errcheck
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, "", fmt.Errorf("failed to fetch URL %s: HTTP %d", url, resp.StatusCode)
+		return nil, "", types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}
 
 	content, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return nil, "", fmt.Errorf("failed to read response body from %s: %w", url, err)
+		return nil, "", types.NewHTTPFetchError(url, err)
 	}
 
 	// Extract filename from URL path