Replace complex SystemRebuild with simple SSH + tmux popup approach

- Remove all SystemRebuild command infrastructure from agent and dashboard
- Replace with direct tmux popup execution: ssh {user}@{host} {alias}
- Add configurable SSH user and rebuild alias in dashboard config
- Eliminate agent process crashes during rebuilds
- Simplify architecture by removing ZMQ command streaming complexity
- Clean up all related dead code and fix compilation warnings

Benefits:
- Process isolation: rebuild runs independently via SSH
- Crash resilience: agent/dashboard can restart without affecting rebuilds
- Configuration flexibility: SSH user and alias configurable per deployment
- Operational simplicity: standard tmux popup interface

agent/src/communication/mod.rs

@@ -1,5 +1,5 @@
 use anyhow::Result;
-use cm_dashboard_shared::{CommandOutputMessage, MessageEnvelope, MetricMessage};
+use cm_dashboard_shared::{MessageEnvelope, MetricMessage};
 use tracing::{debug, info};
 use zmq::{Context, Socket, SocketType};
 
@@ -65,23 +65,6 @@ 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)
 
@@ -122,13 +105,6 @@ pub enum AgentCommand {
         service_name: String,
         action: ServiceAction,
     },
-    /// Rebuild NixOS system
-    SystemRebuild {
-        git_url: String,
-        git_branch: String,
-        working_dir: String,
-        api_key_file: Option<String>,
-    },
 }
 
 /// Service control actions