Abort startup on a config file pixa cannot read (closes #176)

Of the places pixa looks for its config file on its own, it passed over
any place where os.Stat failed, so a file in a directory pixa may not
enter was skipped without a word and pixa started on a later file or on
the environment and defaults. Now only a path that does not exist, or
that runs through a file (such as under a HOME of /dev/null), is passed
over; any other error aborts startup naming the file, as a file that
does not parse already did. README.md says so where it gives the search
order. A config.yml that links to itself tests this as root too.

Model: opus-5-5

internal/config/config.go

@@ -4,6 +4,7 @@ package config
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"log/slog"
 	"math"
 	"net/netip"
@@ -14,6 +15,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 
 	"git.eeqj.de/sneak/smartconfig"
@@ -778,19 +780,27 @@ func loadConfigFile(log *slog.Logger, appName string) (*smartconfig.Config, erro
 	for _, path := range configPaths {
 		cleanPath := filepath.Clean(path)
 
+		// Only a config file that does not exist is skipped, including
+		// one whose path runs through a file, such as under a HOME of
+		// /dev/null. One that cannot be read or does not parse is a
+		// fatal startup error.
 		_, statErr := os.Stat(cleanPath)
-		if statErr == nil {
-			// A config file that exists but does not parse is a fatal
-			// startup error, never something to skip over.
-			sc, err := smartconfig.NewFromConfigPath(path)
-			if err != nil {
-				return nil, fmt.Errorf("failed to parse config file %s: %w", path, err)
-			}
-
-			log.Info("loaded config file", "path", path)
-
-			return sc, nil
+		if errors.Is(statErr, fs.ErrNotExist) || errors.Is(statErr, syscall.ENOTDIR) {
+			continue
 		}
+
+		if statErr != nil {
+			return nil, fmt.Errorf("failed to read config file %s: %w", path, statErr)
+		}
+
+		sc, err := smartconfig.NewFromConfigPath(path)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse config file %s: %w", path, err)
+		}
+
+		log.Info("loaded config file", "path", path)
+
+		return sc, nil
 	}
 
 	return nil, nil //nolint:nilnil // nil config is valid (use defaults)