feat(agent): systemd service install/uninstall subcommands (alpha.11)

For Saturday's Ubuntu host + Linux VM: 'corrosion-host-agent install' writes a
systemd unit (Type=simple — the agent already handles SIGTERM cleanly),
daemon-reloads, and enables+starts the service; 'uninstall' reverses it.

- new service.rs: pure unit_file_contents() generator (unit-tested) + Linux
  install/uninstall via systemctl; non-Linux returns a clear 'Linux only' error
  (Windows SCM is the follow-up).
- ExecStart honors the resolved --config path (default or explicit).
- Runs as root: the agent supervises game processes + their files, needs broad
  filesystem access.

cargo check + service unit test green. Tag agent-v2.0.0-alpha.11 -> CI signs ->
CDN /host-agent/alpha/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

corrosion-host-agent/Cargo.lock

@@ -287,7 +287,7 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
 [[package]]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.10"
+version = "2.0.0-alpha.11"
 dependencies = [
  "anyhow",
  "async-nats",