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/disk.rs

@@ -9,7 +9,7 @@ use std::process::Command;
 use std::time::Instant;
 use tracing::debug;
 
-use super::{Collector, CollectorError, PerformanceMetrics};
+use super::{Collector, CollectorError};
 
 /// Information about a mounted disk
 #[derive(Debug, Clone)]
@@ -22,7 +22,6 @@ struct MountedDisk {
     used: String,            // e.g., "45G"
     available: String,       // e.g., "75G"
     usage_percent: f32,      // e.g., 38.5
-    config_name: Option<String>, // Name from config if UUID-based
 }
 
 /// Disk usage collector for monitoring filesystem sizes
@@ -117,7 +116,6 @@ impl DiskCollector {
                                 used,
                                 available,
                                 usage_percent: usage_percent as f32,
-                                config_name: Some(fs_config.name.clone()),
                             });
 
                             debug!(
@@ -611,7 +609,4 @@ impl Collector for DiskCollector {
         Ok(metrics)
     }
 
-    fn get_performance_metrics(&self) -> Option<PerformanceMetrics> {
-        None // Performance tracking handled by cache system
-    }
 }