fix: use errors.Is for robust file-not-found check

cmd/ingestor/config.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -86,7 +87,7 @@ func LoadConfig(path string) (*Config, error) {
 
 	data, err := os.ReadFile(path)
 	if err != nil {
-		if !os.IsNotExist(err) {
+		if !errors.Is(err, os.ErrNotExist) {
 			return nil, fmt.Errorf("reading config %s: %w", path, err)
 		}
 		// Config file doesn't exist — use defaults (zero-config mode)