Implement real-time terminal popup for system rebuild operations

- Add terminal popup UI component with 80% screen coverage and terminal styling
- Extend ZMQ protocol with CommandOutputMessage for streaming output
- Implement real-time output streaming in agent system rebuild handler
- Add keyboard controls (ESC/Q to close, ↑↓ to scroll) for popup interaction
- Fix system panel Build display to show actual NixOS build instead of config hash
- Update service filters in README with wildcard patterns for better matching
- Add periodic progress updates during nixos-rebuild execution
- Integrate command output handling in dashboard main loop

agent/src/communication/mod.rs

@@ -1,5 +1,5 @@
 use anyhow::Result;
-use cm_dashboard_shared::{MessageEnvelope, MetricMessage};
+use cm_dashboard_shared::{CommandOutputMessage, MessageEnvelope, MetricMessage};
 use tracing::{debug, info};
 use zmq::{Context, Socket, SocketType};
 
@@ -65,6 +65,24 @@ impl ZmqHandler {
         Ok(())
     }
 
+    /// Publish command output message via ZMQ
+    pub async fn publish_command_output(&self, message: &CommandOutputMessage) -> Result<()> {
+        debug!(
+            "Publishing command output for host {} (command: {}): {}",
+            message.hostname,
+            message.command_type,
+            message.output_line
+        );
+
+        let envelope = MessageEnvelope::command_output(message.clone())?;
+        let serialized = serde_json::to_vec(&envelope)?;
+        
+        self.publisher.send(&serialized, 0)?;
+        
+        debug!("Command output published successfully");
+        Ok(())
+    }
+
     /// Send heartbeat (placeholder for future use)
 
     /// Try to receive a command (non-blocking)