Fix two misleading messages (closes #240)

The warning for a config file that others can read always said the
file contained S3 credentials, so a file:// config with none got a
false claim. When s3.access_key_id or s3.secret_access_key is set it
now says the file may contain them, because Load sees the values only
after smartconfig has replaced any ${...} reference, so a set
credential need not be in the file. Otherwise it says the file is
readable by others.

snapshot purge wrapped the listing error, which already starts with
"listing remote snapshots:", in that prefix a second time.
syncWithRemote now returns it unwrapped, as CleanupLocalSnapshots
does.

Model: opus-5-5

internal/config/config.go

@@ -305,7 +305,7 @@ func Load(path string) (*Config, error) {
 	if statErr == nil {
 		mode := info.Mode().Perm()
 		if mode&0044 != 0 { // group or world readable
-			log.Warn("Config file has insecure permissions (contains S3 credentials)",
+			log.Warn(cfg.readableByOthersWarning(),
 				"path", path,
 				"mode", fmt.Sprintf("%04o", mode),
 				"recommendation", "chmod 600 "+path)
@@ -418,6 +418,18 @@ func (c *Config) setAgeSecretKey() {
 	}
 }
 
+// readableByOthersWarning is the warning Load logs when others can read
+// the config file. It says "may contain" because the S3 credentials are
+// seen only after smartconfig has replaced any ${...} reference in the
+// file with its value, so a set credential need not be in the file.
+func (c *Config) readableByOthersWarning() string {
+	if c.S3.AccessKeyID != "" || c.S3.SecretAccessKey != "" {
+		return "Config file is readable by others and may contain S3 credentials"
+	}
+
+	return "Config file is readable by others"
+}
+
 // validateStorage validates storage configuration.
 // If StorageURL is set, it takes precedence. S3 URLs require credentials.
 // File URLs don't require any S3 configuration.