Move SMART configuration into disk config

- Consolidate SMART thresholds into DiskConfig structure
- Remove separate SmartConfig - disk collector handles all drive data
- Update NixOS configuration to use disk.temperature_* settings
- Remove hardcoded temperature thresholds in disk collector
- Logical grouping: disk collector owns all disk/drive configuration

agent/src/collectors/disk.rs

@@ -41,11 +41,11 @@ pub struct DiskCollector {
 
 impl DiskCollector {
     pub fn new(config: DiskConfig) -> Self {
-        // Create hysteresis thresholds for disk temperature
+        // Create hysteresis thresholds for disk temperature from config
         let temperature_thresholds = HysteresisThresholds::with_custom_gaps(
-            60.0, // warning at 60°C
+            config.temperature_warning_celsius,
             5.0,  // 5°C gap for recovery
-            70.0, // critical at 70°C  
+            config.temperature_critical_celsius,
             5.0,  // 5°C gap for recovery
         );