Add Linux systemd support and --install-service/--remove-service flags

The service package now has a Linux implementation alongside the Windows
one: systemd unit install/remove (/etc/systemd/system), readiness
notification (READY=1 via go-systemd) sent only after the listeners are
bound, a 30s watchdog, and STOPPING on shutdown. Listeners are pre-bound
so port conflicts fail fast and the readiness signal is truthful. The
notify calls are no-ops without NOTIFY_SOCKET (containers, shells) and
on non-Linux builds. --install-service/--remove-service work on both
platforms; the 'service install|remove' subcommand remains as an alias.

README.md

@@ -94,15 +94,15 @@ go build ./cmd/gpu-turnstile
 ./gpu-turnstile
 ```
 
-### Run natively on Windows (primary deployment)
+### Run natively on Windows (current primary deployment)
 
 Download `gpu-turnstile.exe` from a release, put a `gpu-turnstile.env`
 next to it, and run it — or install it as a Windows service from an
 elevated shell:
 
 ```sh
-gpu-turnstile.exe service install          # auto-start service, recovery = restart
-gpu-turnstile.exe service remove
+gpu-turnstile.exe --install-service        # auto-start service, recovery = restart
+gpu-turnstile.exe --remove-service
 ```
 
 The service uses the config file (services have no convenient
@@ -115,6 +115,23 @@ the install directory for self-updates. For least privilege, run it as the
 virtual account `NT SERVICE\gpu-turnstile` and grant write access to just
 those two directories.
 
+### Run natively on Linux (systemd)
+
+The same binary works on Linux. Install it as a systemd service as root:
+
+```sh
+gpu-turnstile --install-service            # writes + enables + starts the unit
+gpu-turnstile --remove-service
+```
+
+The unit (`/etc/systemd/system/gpu-turnstile.service`) is `Type=notify`:
+`systemctl start` blocks until the listeners are actually bound, a 30 s
+watchdog restarts the process if it wedges, and logs land in the journal
+(`journalctl -u gpu-turnstile -f`) unless `LOG_FILE` is set. Put the
+config in a `gpu-turnstile.env` next to the binary (or pass
+`-config /path` during install). The notify integration is a no-op in
+containers and interactive shells.
+
 **Auto-update is on by default**: the binary checks the repo's latest
 release on startup and every `UPDATE_INTERVAL`, verifies the Ed25519
 signature of the download against the public key embedded at build time,