feat(host-agent): Phase 1a process supervision — instance start/stop/restart/status + push state events

Per-instance ProcessSupervisor: tokio child spawn with proper arg list
(fixes Go's naive space-splitting), graceful SIGTERM with 30s budget
then force kill, monitor task classifying ordered-stop vs crash (exit
code captured), watch-channel state observable everywhere. Instance cmd
channel live on corrosion.{license}.{instance}.cmd (start/stop/restart/
status) with state events pushed on {instance}.status (keep-latest
semantics, documented). Heartbeats now carry live process state +
uptime per instance. Crate restructured lib+bin for integration tests.

Verified: 5 integration tests with real OS processes (lifecycle, crash
exit-code, restart recovery, unmanaged rejection, clean spawn failure)
+ live-NATS contract test (request-reply roundtrips, double-start
rejection, push events, heartbeat state) — all green.

Known limitation (documented): no PID adoption yet — agent restart
orphans a running game process to 'stopped' until panel restart.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/src/config.rs

@@ -49,6 +49,29 @@ pub struct InstanceConfig {
     /// Optional human label shown in the panel.
     #[serde(default)]
     pub label: Option<String>,
+    /// Game server executable. Relative paths resolve against `root`.
+    /// Absent = unmanaged instance (telemetry only, no process control).
+    #[serde(default)]
+    pub executable: Option<PathBuf>,
+    /// Arguments as a proper list — no shell splitting, quoted values survive.
+    #[serde(default)]
+    pub args: Vec<String>,
+    /// Working directory for the process. Defaults to the executable's directory.
+    #[serde(default)]
+    pub working_dir: Option<PathBuf>,
+}
+
+impl InstanceConfig {
+    /// Absolute executable path, if this instance is process-managed.
+    pub fn resolved_executable(&self) -> Option<PathBuf> {
+        self.executable.as_ref().map(|exe| {
+            if exe.is_absolute() {
+                exe.clone()
+            } else {
+                self.root.join(exe)
+            }
+        })
+    }
 }
 
 #[derive(Debug, Clone, Default, Deserialize)]