Remove hardcoded defaults, require configuration file

- Remove all Default implementations from agent configuration structs
- Make configuration file required for agent startup
- Update NixOS module to generate complete agent.toml configuration
- Add comprehensive configuration options to NixOS module including:
  - Service include/exclude patterns for systemd collector
  - All thresholds and intervals
  - ZMQ communication settings
  - Notification and cache configuration
- Agent now fails fast if no configuration provided
- Eliminates configuration drift between defaults and NixOS settings

agent/src/collectors/systemd.rs

@@ -128,8 +128,19 @@ impl SystemdCollector {
             .arg("--plain")
             .output()?;
 
-        // Also get user unit files (user-level services)
-        let user_unit_files_output = Command::new("systemctl")
+        // Get hostname to determine which user to check
+        let hostname = gethostname::gethostname().to_string_lossy().to_string();
+        let target_user = match hostname.as_str() {
+            "steambox" | "cmbox" => "cm",
+            "simonbox" => "simon",
+            _ => "cm", // default to cm for unknown hosts
+        };
+
+        // Also get user unit files (user-level services) for target user
+        let user_unit_files_output = Command::new("sudo")
+            .arg("-u")
+            .arg(target_user)
+            .arg("systemctl")
             .arg("--user")
             .arg("list-unit-files")
             .arg("--type=service")
@@ -137,8 +148,11 @@ impl SystemdCollector {
             .arg("--plain")
             .output()?;
 
-        // And user loaded units
-        let user_units_output = Command::new("systemctl")
+        // And user loaded units for target user
+        let user_units_output = Command::new("sudo")
+            .arg("-u")
+            .arg(target_user)
+            .arg("systemctl")
             .arg("--user")
             .arg("list-units")
             .arg("--type=service")