chore(config): remove unused Config fields with no readers (#836)

These fields are not referenced anywhere in node-agent — nothing reads them,
and removing them (plus their viper defaults) has no effect on behavior. This
trims the Config struct and the documented config surface.

Removed: backendStorageEnabled, hostMalwareSensorEnabled,
hostNetworkSensorEnabled, testMode.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>