complete fgj → fj rename: env vars, config migration, docs

- Rename env vars: FGJ_HOST → FJ_HOST, FGJ_TOKEN → FJ_TOKEN,
  FGJ_FORCE_TTY → FJ_FORCE_TTY, FGJ_PAGER → FJ_PAGER,
  FGJ_BINARY_PATH → FJ_BINARY_PATH (all with legacy fallback)
- Auto-migrate ~/.config/fgj/ → ~/.config/fj/ on first run
- Update man page title, README, CHANGELOG
- Update test fixture labels from [FGJ E2E Test] to [FJ E2E Test]

cmd/auth.go

@@ -188,7 +188,7 @@ func resolveAuthHostname(cfg *config.Config, hostname string) (string, error) {
 		hostname = viper.GetString("hostname")
 	}
 	if hostname == "" {
-		hostname = os.Getenv("FGJ_HOST")
+		hostname = config.EnvWithFallback("FJ_HOST", "FGJ_HOST")
 	}
 	if hostname == "" {
 		hostname = getDetectedHost()