Switch config format from YAML to TOML

Replaces gopkg.in/yaml.v3 with github.com/BurntSushi/toml. Updates
struct tags, default config path, Dockerfile CMD, README, and ships
config.example.toml in place of the YAML example. ${ENV_VAR}
expansion still runs on the raw file before parsing, so the behavior
is unchanged.

main.go

@@ -15,7 +15,7 @@ import (
 )
 
 func main() {
-	configPath := flag.String("config", "config.yaml", "path to config file")
+	configPath := flag.String("config", "config.toml", "path to config file")
 	flag.Parse()
 
 	slog.SetDefault(slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelInfo})))