feat: Add Oxide/uMod installer package + wire into companion agent daemon

New oxide package downloads latest Oxide.Rust release from GitHub,
extracts over the server directory, and restarts the game server.
Progress published to NATS (corrosion.{license_id}.oxide.status).
Heartbeat now reports oxide_installed status.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

companion-agent/internal/oxide/status.go

@@ -0,0 +1,31 @@
+package oxide
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// OxideStatus represents a progress update published to NATS during Oxide installation.
+// The frontend listens on corrosion.{license_id}.oxide.status for these messages.
+type OxideStatus struct {
+	Stage     string `json:"stage"`
+	Progress  int    `json:"progress"`
+	Message   string `json:"message"`
+	Error     string `json:"error,omitempty"`
+	Timestamp string `json:"timestamp"`
+}
+
+// Valid installation stages:
+//   fetching_release - Querying GitHub API for latest Oxide.Rust release
+//   downloading      - Downloading the Oxide zip file
+//   installing       - Extracting zip over server directory
+//   restarting       - Restarting the game server to load Oxide
+//   complete         - Oxide installation finished successfully
+//   failed           - Installation failed at some stage
+
+// CheckOxideInstalled returns true if the oxide/ directory exists in the
+// server installation directory, indicating that Oxide/uMod has been installed.
+func CheckOxideInstalled(installDir string) bool {
+	_, err := os.Stat(filepath.Join(installDir, "server", "oxide"))
+	return err == nil
+}