🐛 fix: emit all config-loading logs in correct format from the start

The logger was initialised to console format unconditionally, so the
"Config file loaded" message (and similar early logs) were always written
as human-readable text even when JSON logging was configured.

Root cause: classic chicken-and-egg — the format flag lives inside the
config that is being loaded.

Fix: add peekJSONLogging() which resolves the format before any log is
emitted by (1) checking DLC_LOGGING_JSON directly via os.Getenv, then
(2) doing a minimal throwaway Viper pre-read of the config file for the
logging.json key.  The redundant format-switch block that ran after
Unmarshal() is removed.

Also add the "Logging configured" log as the very first line, and
replace the hardcoded PROJECT_DIR path in start-server.sh,
test-graceful-shutdown.sh, and test-opentelemetry.sh with a dynamic
derivation from BASH_SOURCE[0].

Closes #15

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/config/config.go

@@ -118,6 +118,34 @@ type SamplerConfig struct {
 	Ratio float64 `mapstructure:"ratio"`
 }
 
+// peekJSONLogging determines whether JSON logging should be used before the full
+// config is loaded, solving the chicken-and-egg problem where the logger format
+// must be known before any log is emitted, yet the format is stored in the config.
+//
+// Resolution order (mirrors Viper's own priority):
+//  1. DLC_LOGGING_JSON env var — checked directly via os.Getenv (zero overhead)
+//  2. logging.json key in the config file — read with a minimal throwaway Viper
+//     instance so we don't parse the whole config twice unnecessarily
+func peekJSONLogging() bool {
+	// 1. Env var takes highest priority — check it first
+	if env := os.Getenv("DLC_LOGGING_JSON"); env != "" {
+		return strings.EqualFold(env, "true") || env == "1"
+	}
+
+	// 2. Try to read logging.json from the config file
+	preV := viper.New()
+	preV.SetDefault("logging.json", false)
+	if configFile := os.Getenv("DLC_CONFIG_FILE"); configFile != "" {
+		preV.SetConfigFile(configFile)
+	} else {
+		preV.SetConfigName("config")
+		preV.SetConfigType("yaml")
+		preV.AddConfigPath(".")
+	}
+	_ = preV.ReadInConfig() // ignore errors — defaults apply on failure
+	return preV.GetBool("logging.json")
+}
+
 // LoadConfig loads configuration from file, environment variables, and defaults
 // Configuration priority: file > environment variables > defaults
 // To specify a custom config file path, set DLC_CONFIG_FILE environment variable
@@ -129,9 +157,17 @@ func LoadConfig() (*Config, error) {
 
 	v := viper.New()
 
-	// Set up initial console logging for config loading messages
-	consoleWriter := zerolog.ConsoleWriter{Out: os.Stderr}
-	log.Logger = log.Output(consoleWriter)
+	// Configure the logger format before emitting any log output.
+	// peekJSONLogging reads the JSON setting early (env var + config file pre-read)
+	// so that every log line — including those produced during config loading — is
+	// already in the correct format.
+	jsonLogging := peekJSONLogging()
+	if jsonLogging {
+		log.Logger = log.Output(os.Stderr)
+	} else {
+		log.Logger = log.Output(zerolog.ConsoleWriter{Out: os.Stderr})
+	}
+	log.Info().Bool("json", jsonLogging).Msg("Logging configured")
 
 	// Set default values
 	v.SetDefault("server.host", "0.0.0.0")
@@ -227,15 +263,9 @@ func LoadConfig() (*Config, error) {
 		return nil, fmt.Errorf("config unmarshal error: %w", err)
 	}
 
-	// Configure log output format (JSON or console) first
-	if config.Logging.JSON {
-		log.Logger = log.Output(os.Stderr)
-	} else {
-		consoleWriter := zerolog.ConsoleWriter{Out: os.Stderr}
-		log.Logger = log.Output(consoleWriter)
-	}
-
-	// Setup logging based on configuration
+	// Setup logging based on configuration (level, output file, time format).
+	// The JSON/console format was already applied at the top of LoadConfig via
+	// peekJSONLogging, so SetupLogging only needs to handle the remaining knobs.
 	config.SetupLogging()
 
 	log.Info().