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

CLAUDE.md

@@ -311,8 +311,12 @@ Agent → ["cpu_load_1min", "memory_usage_percent", ...] → Dashboard → Widge
 - CLI: `cm-dashboard-agent [-v]` (intelligent caching enabled)
 
 **Service Discovery:**
-- Scans running systemd services
-- Filters by predefined interesting patterns (gitea, nginx, docker, etc.)
+- Scans ALL systemd services (active, inactive, failed, dead, etc.) using list-unit-files and list-units --all
+- Discovers both system services and user services per host:
+  - steambox/cmbox: reads system + cm user services  
+  - simonbox: reads system + simon user services
+- Filters by service_name_filters patterns (gitea, nginx, docker, sunshine, etc.)
+- Excludes maintenance services (docker-prune, sshd@, ark-permissions, etc.)
 - No host-specific hardcoded service lists
 
 ### Current Implementation Status