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/src/main.rs

@@ -6,7 +6,7 @@
 
 use corrosion_host_agent::{
     agent, bus, config, docker_compose, filemanager, hostcmd, instancecmd, prober, process,
-    subjects, supervisor, telemetry, version,
+    service, subjects, supervisor, telemetry, version,
 };
 
 use anyhow::{Context, Result};
@@ -37,6 +37,10 @@ enum Command {
     Check,
     /// Print full version (semver, git hash, build timestamp) and exit.
     Version,
+    /// Install as a systemd service and start it (Linux; requires root).
+    Install,
+    /// Stop and remove the systemd service (Linux; requires root).
+    Uninstall,
 }
 
 fn main() -> Result<()> {
@@ -58,6 +62,8 @@ fn main() -> Result<()> {
             );
             Ok(())
         }
+        Some(Command::Install) => service::install(&config_path),
+        Some(Command::Uninstall) => service::uninstall(),
         None => {
             let settings = config::load(&config_path)?;
             init_logging(&settings.log_level);