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

dashboard/src/config/mod.rs

@@ -8,6 +8,7 @@ pub struct DashboardConfig {
     pub zmq: ZmqConfig,
     pub hosts: HostsConfig,
     pub system: SystemConfig,
+    pub ssh: SshConfig,
 }
 
 /// ZMQ consumer configuration
@@ -31,6 +32,13 @@ pub struct SystemConfig {
     pub nixos_config_api_key_file: Option<String>,
 }
 
+/// SSH configuration for rebuild operations
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SshConfig {
+    pub rebuild_user: String,
+    pub rebuild_alias: String,
+}
+
 impl DashboardConfig {
     pub fn load_from_file<P: AsRef<Path>>(path: P) -> Result<Self> {
         let path = path.as_ref();