docs: document scoped systemd service names for multi-install

- Update messaging guide to use 'hermes gateway' CLI commands instead
  of raw systemctl (auto-resolves the correct service name)
- Add info callout explaining multi-install service name scoping
- Update HERMES_HOME env var docs to mention PID + service name scoping

website/docs/user-guide/messaging/index.md

@@ -244,10 +244,10 @@ Background tasks on messaging platforms are fire-and-forget — you don't need t
 
 ```bash
 hermes gateway install               # Install as user service
-systemctl --user start hermes-gateway
-systemctl --user stop hermes-gateway
-systemctl --user status hermes-gateway
-journalctl --user -u hermes-gateway -f
+hermes gateway start                 # Start the service
+hermes gateway stop                  # Stop the service
+hermes gateway status                # Check status
+journalctl --user -u hermes-gateway -f  # View logs
 
 # Enable lingering (keeps running after logout)
 sudo loginctl enable-linger $USER
@@ -263,6 +263,10 @@ Use the user service on laptops and dev boxes. Use the system service on VPS or
 
 Avoid keeping both the user and system gateway units installed at once unless you really mean to. Hermes will warn if it detects both because start/stop/status behavior gets ambiguous.
 
+:::info Multiple installations
+If you run multiple Hermes installations on the same machine (with different `HERMES_HOME` directories), each gets its own systemd service name. The default `~/.hermes` uses `hermes-gateway`; other installations use `hermes-gateway-<hash>`. The `hermes gateway` commands automatically target the correct service for your current `HERMES_HOME`.
+:::
+
 ### macOS (launchd)
 
 ```bash