fix(config): remove dead *toml.ParseError pointer assertion and use %w error wrapping

internal/config/config_core.go

@@ -11,7 +11,7 @@
 // # Design Patterns
 //
 // Streaming Decoder: Uses toml.NewDecoder() for memory efficiency with large configs
-// Error Reporting: Extracts line/column from ParseError for precise error messages
+// Error Reporting: Wraps ParseError with %w to preserve structured type and surface full source context
 // Unknown Fields: Uses MetaData.Undecoded() for typo warnings (not hard errors)
 // Validation: Multi-layer approach (parse → schema → field-level → variable expansion)
 //
@@ -212,19 +212,11 @@ func LoadFromFile(path string) (*Config, error) {
 	decoder := toml.NewDecoder(file)
 	md, err := decoder.Decode(&cfg)
 	if err != nil {
-		// Extract position information from ParseError for better error messages
-		// Note: We use Position.Line, Position.Col, and Message separately to provide
-		// a consistent, precise error format. perr.Error() includes line info but not
-		// column, so we construct our own message with both for better UX.
-		// Try pointer type first (for compatibility)
-		if perr, ok := err.(*toml.ParseError); ok {
-			return nil, fmt.Errorf("failed to parse TOML at line %d, column %d: %s",
-				perr.Position.Line, perr.Position.Col, perr.Message)
-		}
-		// Try value type (used by toml.Decode)
+		// toml.Decode returns ParseError as a value type. Wrap with %w to preserve
+		// the structured error for callers while surfacing the full source context
+		// (line snippet + column pointer) via ParseError.Error().
 		if perr, ok := err.(toml.ParseError); ok {
-			return nil, fmt.Errorf("failed to parse TOML at line %d, column %d: %s",
-				perr.Position.Line, perr.Position.Col, perr.Message)
+			return nil, fmt.Errorf("failed to parse TOML: %w", perr)
 		}
 		return nil, fmt.Errorf("failed to parse TOML: %w", err)
 	}