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/ui/widgets/system.rs

@@ -129,7 +129,7 @@ impl SystemWidget {
     }
 
     /// Get the current agent hash for rebuild completion detection
-    pub fn get_agent_hash(&self) -> Option<&String> {
+    pub fn _get_agent_hash(&self) -> Option<&String> {
         self.agent_hash.as_ref()
     }