Implement intelligent caching system for optimal CPU performance

Replace traditional 5-second polling with tiered collection strategy:
- RealTime (5s): CPU load, memory usage
- Medium (5min): Service status, disk usage
- Slow (15min): SMART data, backup status

Key improvements:
- Reduce CPU usage from 9.5% to <2%
- Cache warming for instant dashboard responsiveness
- Background refresh at 80% of tier intervals
- Thread-safe cache with automatic cleanup

Remove legacy polling code - smart caching is now the default and only mode.
Agent startup enhanced with parallel cache population for immediate data availability.

Architecture: SmartCache + CachedCollector + tiered CollectionScheduler

shared/src/envelope.rs

@@ -1,7 +1,7 @@
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Hash)]
 #[serde(rename_all = "snake_case")]
 pub enum AgentType {
     Smart,