🐛 fix: emit all config-loading logs in correct JSON format from the start (#16)

## Summary

Closes #15

When `logging.json: true` (or `DLC_LOGGING_JSON=true`), the logger was unconditionally initialised to console/text format at the top of `LoadConfig()`, so early log lines — most visibly **"Config file loaded"** — were always written as human-readable text regardless of configuration.

## Root cause

Classic chicken-and-egg: the format flag lives inside the config that is being loaded. The format-switch block only ran *after* `v.Unmarshal()`, too late for the config-file log.

## Changes

### `pkg/config/config.go`
- Add `peekJSONLogging()`: resolves the JSON flag **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. This mirrors Viper's own priority order without parsing the full config twice.
- Apply the resolved format immediately and emit **"Logging configured"** as the very first log line.
- Remove the now-redundant format-switch block that ran after `Unmarshal()`.

### `scripts/start-server.sh`, `test-graceful-shutdown.sh`, `test-opentelemetry.sh`
- Replace hardcoded `PROJECT_DIR` path with a dynamic `SCRIPTS_DIR=$(dirname $(realpath ${BASH_SOURCE[0]}))` derivation so scripts work from any worktree or clone location.

## Test plan
- [x] `go test ./pkg/...` — all pass
- [x] `scripts/test-graceful-shutdown.sh` — all JSON valid, all startup logs present
- [x] Manual smoke test: first line is `{"level":"info",...,"message":"Logging configured"}`, every line is valid JSON

Reviewed-on: #16
Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.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().