Remove unused code and eliminate compiler warnings

- Remove unused fields from CommandStatus variants
- Clean up unused methods and unused collector fields
- Fix lifetime syntax warning in SystemWidget
- Delete unused cache module completely
- Remove redundant render methods from widgets

All agent and dashboard warnings eliminated while preserving
panel switching and scrolling functionality.

dashboard/src/ui/widgets/system.rs

@@ -249,7 +249,7 @@ impl SystemWidget {
     }
 
     /// Render storage section with tree structure
-    fn render_storage(&self) -> Vec<Line> {
+    fn render_storage(&self) -> Vec<Line<'_>> {
         let mut lines = Vec::new();
 
         for pool in &self.storage_pools {
@@ -410,9 +410,6 @@ impl Widget for SystemWidget {
         self.update_storage_from_metrics(metrics);
     }
 
-    fn render(&mut self, frame: &mut Frame, area: Rect) {
-        self.render_with_scroll(frame, area, 0);
-    }
 }
 
 impl SystemWidget {