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/config/config.go

@@ -638,10 +638,16 @@ func parseConfigFile(configFile types.ConfigFile, maxDepth int) (*Config, error)
 //   - Escaped "@": strings starting with "@@" (literal "@" at start)
 //   - File reference: "@/path/to/file" or "@./relative/path"
 //   - URL reference: "@http://..." or "@https://..."
+//
+// It can return the following errors:
+//   - types.ErrScriptEmpty
+//   - types.ErrScriptSourceEmpty
+//   - types.ErrScriptURLNoHost
+//   - types.URLParseError
 func validateScriptSource(script string) error {
 	// Empty script is invalid
 	if script == "" {
-		return errors.New("script cannot be empty")
+		return types.ErrScriptEmpty
 	}
 
 	// Not a file/URL reference - it's an inline script
@@ -658,17 +664,17 @@ func validateScriptSource(script string) error {
 	source := script[1:] // Remove the @ prefix
 
 	if source == "" {
-		return errors.New("script source cannot be empty after @")
+		return types.ErrScriptSourceEmpty
 	}
 
-	// Check if it's a URL
+	// Check if it's a http(s) URL
 	if strings.HasPrefix(source, "http://") || strings.HasPrefix(source, "https://") {
 		parsedURL, err := url.Parse(source)
 		if err != nil {
-			return fmt.Errorf("invalid URL: %w", err)
+			return types.NewURLParseError(source, err)
 		}
 		if parsedURL.Host == "" {
-			return errors.New("URL must have a host")
+			return types.ErrScriptURLNoHost
 		}
 		return nil
 	}

internal/config/file.go

@@ -49,6 +49,10 @@ func (parser ConfigFileParser) Parse() (*Config, error) {
 }
 
 // fetchFile retrieves file contents from a local path or HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.FileReadError
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func fetchFile(ctx context.Context, src string) ([]byte, error) {
 	if strings.HasPrefix(src, "http://") || strings.HasPrefix(src, "https://") {
 		return fetchHTTP(ctx, src)
@@ -57,25 +61,28 @@ func fetchFile(ctx context.Context, src string) ([]byte, error) {
 }
 
 // fetchHTTP downloads file contents from an HTTP/HTTPS URL.
+// It can return the following errors:
+//   - types.HTTPFetchError
+//   - types.HTTPStatusError
 func fetchHTTP(ctx context.Context, url string) ([]byte, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
-		return nil, fmt.Errorf("failed to create request: %w", err)
+		return nil, types.NewHTTPFetchError(url, err)
 	}
 
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
-		return nil, fmt.Errorf("failed to fetch file: %w", 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 file: HTTP %d %s", resp.StatusCode, resp.Status)
+		return nil, types.NewHTTPStatusError(url, resp.StatusCode, resp.Status)
 	}
 
 	data, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read response body: %w", err)
+		return nil, types.NewHTTPFetchError(url, err)
 	}
 
 	return data, nil
@@ -83,19 +90,21 @@ func fetchHTTP(ctx context.Context, url string) ([]byte, error) {
 
 // fetchLocal reads file contents from the local filesystem.
 // It resolves relative paths from the current working directory.
+// It can return the following errors:
+//   - types.FileReadError
 func fetchLocal(src string) ([]byte, error) {
 	path := src
 	if !filepath.IsAbs(src) {
 		pwd, err := os.Getwd()
 		if err != nil {
-			return nil, fmt.Errorf("failed to get working directory: %w", err)
+			return nil, types.NewFileReadError(src, err)
 		}
 		path = filepath.Join(pwd, src)
 	}
 
 	data, err := os.ReadFile(path) //nolint:gosec
 	if err != nil {
-		return nil, fmt.Errorf("failed to read file: %w", err)
+		return nil, types.NewFileReadError(path, err)
 	}
 
 	return data, nil

internal/config/template_validator.go

@@ -8,6 +8,8 @@ import (
 	"go.aykhans.me/sarin/internal/types"
 )
 
+// It can return the following errors:
+//   - types.TemplateParseError
 func validateTemplateString(value string, funcMap template.FuncMap) error {
 	if value == "" {
 		return nil
@@ -15,7 +17,7 @@ func validateTemplateString(value string, funcMap template.FuncMap) error {
 
 	_, err := template.New("").Funcs(funcMap).Parse(value)
 	if err != nil {
-		return fmt.Errorf("template parse error: %w", err)
+		return types.NewTemplateParseError(err)
 	}
 
 	return nil