feat: Add Go companion agent for bare metal server management

Implements complete companion agent for Rust servers not on managed panels.

Features:
- NATS integration with token auth and auto-reconnect
- Game server process management (start/stop/restart/monitor)
- File operations (read/write/delete/list) via NATS
- SteamCMD integration for automated updates
- Self-update capability with download and replace
- Heartbeat publishing every 60s with server status
- Graceful shutdown handling (SIGTERM/SIGINT)
- Zombie process prevention via cmd.Wait()
- Cross-platform builds (Linux amd64, Windows amd64)

Structure:
- cmd/agent/main.go: Entry point, config, signal handling
- internal/app/daemon.go: Main loop, NATS subscriptions
- internal/client/nats.go: NATS connection with reconnect
- internal/process/gameserver.go: Process management
- internal/process/steamcmd.go: Steam update execution
- internal/files/operations.go: File system operations
- internal/update/updater.go: Self-update logic
- Makefile: Cross-compilation targets
- README.md: Installation and configuration guide

NATS Subjects:
- Publishes: corrosion.{license_id}.companion.heartbeat
- Publishes: corrosion.{license_id}.files.response
- Subscribes: corrosion.{license_id}.cmd.server
- Subscribes: corrosion.{license_id}.files.{get|put|delete|list}
- Subscribes: corrosion.{license_id}.update.steam
- Subscribes: corrosion.{license_id}.update.companion

Built binaries: 7.0MB (Linux), 7.2MB (Windows)
Total code: 1,356 LOC across 8 files

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

companion-agent/cmd/agent/main.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"context"
+	"log"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/kelseyhightower/envconfig"
+	"github.com/vigilcyber/corrosion-companion/internal/app"
+	"github.com/vigilcyber/corrosion-companion/internal/client"
+)
+
+// Config holds all environment-based configuration
+type Config struct {
+	// NATS connection
+	NATSUrl   string `envconfig:"NATS_URL" required:"true"`
+	NATSToken string `envconfig:"NATS_TOKEN" required:"true"`
+
+	// License identification
+	LicenseID string `envconfig:"LICENSE_ID" required:"true"`
+
+	// Game server configuration
+	SteamCMDPath   string `envconfig:"STEAMCMD_PATH" default:"/usr/games/steamcmd"`
+	GameServerPath string `envconfig:"GAME_SERVER_PATH" required:"true"`
+	GameServerArgs string `envconfig:"GAME_SERVER_ARGS" default:"-batchmode"`
+
+	// Optional settings
+	HeartbeatInterval int `envconfig:"HEARTBEAT_INTERVAL" default:"60"`
+	LogLevel          string `envconfig:"LOG_LEVEL" default:"info"`
+}
+
+const version = "1.0.0"
+
+func main() {
+	log.SetFlags(log.LstdFlags | log.Lshortfile)
+	log.Printf("Corrosion Companion Agent v%s starting...", version)
+
+	// Load configuration from environment
+	var cfg Config
+	if err := envconfig.Process("", &cfg); err != nil {
+		log.Fatalf("Failed to load configuration: %v", err)
+	}
+
+	log.Printf("Configuration loaded:")
+	log.Printf("  NATS URL: %s", cfg.NATSUrl)
+	log.Printf("  License ID: %s", cfg.LicenseID)
+	log.Printf("  Game Server Path: %s", cfg.GameServerPath)
+	log.Printf("  SteamCMD Path: %s", cfg.SteamCMDPath)
+	log.Printf("  Heartbeat Interval: %ds", cfg.HeartbeatInterval)
+
+	// Create context with signal handling for graceful shutdown
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer cancel()
+
+	// Connect to NATS
+	log.Println("Connecting to NATS...")
+	nc, err := client.Connect(cfg.NATSUrl, cfg.NATSToken)
+	if err != nil {
+		log.Fatalf("Failed to connect to NATS: %v", err)
+	}
+	defer nc.Close()
+
+	log.Println("NATS connection established")
+
+	// Initialize daemon configuration
+	daemonCfg := &app.DaemonConfig{
+		LicenseID:         cfg.LicenseID,
+		HeartbeatInterval: time.Duration(cfg.HeartbeatInterval) * time.Second,
+		SteamCMDPath:      cfg.SteamCMDPath,
+		GameServerPath:    cfg.GameServerPath,
+		GameServerArgs:    cfg.GameServerArgs,
+		Version:           version,
+	}
+
+	// Start daemon
+	daemon, err := app.NewDaemon(nc, daemonCfg)
+	if err != nil {
+		log.Fatalf("Failed to initialize daemon: %v", err)
+	}
+
+	log.Println("Daemon initialized, starting main loop...")
+
+	// Run daemon until context is cancelled
+	if err := daemon.Run(ctx); err != nil {
+		log.Fatalf("Daemon error: %v", err)
+	}
+
+	log.Println("Companion agent shutdown complete")
+}