Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/config/load.go

@@ -68,14 +68,14 @@ func loadConfig(cfgFile string) error {
 		if _, ok := err.(viper.ConfigFileNotFoundError); ok {
 			// Indicates an explicitly set config file is not found or
 			// that no config file was found in any search path
-			slog.Warn("No config file found!")
+			slog.Warn("no config file found; using defaults")
 		} else {
 			// Config file was found but another error was produced
 			return err
 		}
 	} else {
 		// Config file found and successfully parsed
-		slog.Info("Using config file: " + viper.ConfigFileUsed())
+		slog.Info("using config file", "file", viper.ConfigFileUsed())
 	}
 	return nil
 }