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.

examples/service_manager/basic_usage.rhai

@@ -1,13 +1,17 @@
 // Basic Service Manager Usage Example
 //
 // This example demonstrates the basic API of the service manager.
-// It works on both macOS (launchctl) and Linux (zinit).
+// It works on both macOS (launchctl) and Linux (zinit/systemd).
 //
 // Prerequisites:
 //
-// Linux: Make sure zinit is running:
+// Linux: The service manager will automatically discover running zinit servers
+//        or fall back to systemd. To use zinit, start it with:
 //   zinit -s /tmp/zinit.sock init
 //
+//   You can also specify a custom socket path:
+//   export ZINIT_SOCKET_PATH=/your/custom/path/zinit.sock
+//
 // macOS: No additional setup required (uses launchctl).
 //
 // Usage: