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

CLAUDE.md

@@ -33,11 +33,25 @@ All keyboard navigation and service selection features successfully implemented:
 - Service selection cursor implemented with focus-aware highlighting ✅
 - Panel scrolling fixed for System, Services, and Backup panels ✅
 - Build display working: "Build: 25.05.20251004.3bcc93c" ✅
-- Agent version display working: "Agent: 3kvc03nd" ✅
+- Agent version display working: "Agent: v0.1.14" ✅
 - Cross-host version comparison implemented ✅
 - Automated binary release system working ✅
 - SMART data consolidated into disk collector ✅
 
+**CRITICAL ISSUE - Remote Rebuild Functionality:**
+- ❌ **System Rebuild**: Agent crashes during nixos-rebuild operations
+- ❌ **Systemd Service**: cm-rebuild.service fails with exit status 1
+- ❌ **Output Streaming**: Terminal popup shows agent messages but not rebuild output
+- ⚠️ **Service Control**: Works correctly for start/stop/restart of services
+
+**Problem Details:**
+- Implemented systemd service approach to prevent agent crashes
+- Terminal popup implemented with real-time streaming capability
+- Service produces empty journal lines then exits with status 1
+- Permission issues addressed by moving working directory to /tmp
+- Issue persists despite multiple troubleshooting attempts
+- Manual rebuilds work perfectly when done directly
+
 **Current Layout:**
 ```
 NixOS: