Add refresh shortkey 'r' for on-demand metrics refresh

Implements ZMQ command protocol for dashboard-to-agent communication:
- Agents listen on port 6131 for REQ/REP commands
- Dashboard sends "refresh" command when 'r' key is pressed
- Agents force immediate collection of all metrics via force_refresh_all()
- Fresh data is broadcast immediately to dashboard
- Updated help text to show "r: Refresh all metrics"

Also includes metric-level caching architecture foundation for future
granular control over individual metric update frequencies.

dashboard/src/main.rs

@@ -153,6 +153,12 @@ fn run_app(
 
     while !app.should_quit() {
         drain_app_events(app, event_rx);
+        
+        // Check for refresh requests
+        if app.check_refresh_request() {
+            send_refresh_commands(app)?;
+        }
+        
         terminal.draw(|frame| ui::render(frame, app))?;
 
         if event::poll(tick_rate)? {
@@ -301,6 +307,58 @@ fn metrics_blocking_loop(
     Ok(())
 }
 
+fn send_refresh_commands(app: &mut App) -> Result<()> {
+    let endpoints = app.zmq_endpoints();
+    if endpoints.is_empty() {
+        return Ok(());
+    }
+    
+    let zmq_context = NativeZmqContext::new();
+    
+    for endpoint in endpoints {
+        // Convert metrics endpoint (6130) to command endpoint (6131)
+        let command_endpoint = endpoint.replace(":6130", ":6131");
+        
+        let socket = zmq_context.socket(zmq::REQ)?;
+        socket.set_linger(0)?;
+        socket.set_rcvtimeo(5000)?; // 5 second timeout
+        socket.set_sndtimeo(5000)?; // 5 second timeout
+        
+        match socket.connect(&command_endpoint) {
+            Ok(()) => {
+                debug!("Sending refresh command to {}", command_endpoint);
+                
+                match socket.send("refresh", 0) {
+                    Ok(()) => {
+                        // Wait for response
+                        match socket.recv_string(0) {
+                            Ok(Ok(response)) => {
+                                debug!("Refresh response from {}: {}", command_endpoint, response);
+                                // Update status via public method would be needed, for now just log
+                                debug!("Refresh sent to agents - response: {}", response);
+                            }
+                            Ok(Err(e)) => {
+                                warn!("String conversion error from {}: {:?}", command_endpoint, e);
+                            }
+                            Err(e) => {
+                                warn!("No response from {}: {}", command_endpoint, e);
+                            }
+                        }
+                    }
+                    Err(e) => {
+                        warn!("Failed to send refresh to {}: {}", command_endpoint, e);
+                    }
+                }
+            }
+            Err(e) => {
+                warn!("Failed to connect to command endpoint {}: {}", command_endpoint, e);
+            }
+        }
+    }
+    
+    Ok(())
+}
+
 fn handle_zmq_message(
     message: &NativeZmqMessage,
     sender: &UnboundedSender<AppEvent>,