Remove all unused code and fix build warnings

- Remove unused struct fields: tier, config_name, last_collection_time
- Remove unused structs: PerformanceMetrics, PerfMonitor
- Remove unused methods: get_performance_metrics, get_collector_names, get_stats
- Remove unused utility functions and system helpers
- Remove unused config fields from CPU and Memory collectors
- Keep config fields that are actually used (DiskCollector, etc.)
- Remove unused proxy_pass_url variable and assignments
- Fix duplicate hostname variable declaration
- Achieve zero build warnings without functionality changes

agent/src/collectors/cpu.rs

@@ -15,7 +15,6 @@ use crate::config::CpuConfig;
 /// - No process spawning
 /// - <0.1ms collection time target
 pub struct CpuCollector {
-    config: CpuConfig,
     name: String,
     load_thresholds: HysteresisThresholds,
     temperature_thresholds: HysteresisThresholds,
@@ -35,7 +34,6 @@ impl CpuCollector {
         );
         
         Self {
-            config,
             name: "cpu".to_string(),
             load_thresholds,
             temperature_thresholds,
@@ -243,7 +241,4 @@ impl Collector for CpuCollector {
         Ok(metrics)
     }
 
-    fn get_performance_metrics(&self) -> Option<super::PerformanceMetrics> {
-        None // Performance tracking handled by cache system
-    }
 }