feat: Enhance service manager with zinit socket discovery and systemd fallback

- Improve Linux support by automatically discovering zinit sockets
  using environment variables and common paths.
- Add fallback to systemd if no zinit server is detected.
- Enhance README with detailed instructions for zinit usage,
  including custom socket path configuration.
- Add example demonstrating zinit socket discovery.
- Add logging to show socket discovery process.
- Add unit tests for service manager creation and socket discovery.

service_manager/examples/service_spaghetti.rs

@@ -10,6 +10,9 @@ use std::thread;
 use std::time::Duration;
 
 fn main() {
+    // Initialize logging to see socket discovery in action
+    env_logger::init();
+
     let manager = match create_service_manager() {
         Ok(manager) => manager,
         Err(e) => {