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/go.mod

@@ -0,0 +1,16 @@
+module github.com/vigilcyber/corrosion-companion
+
+go 1.21
+
+require (
+	github.com/kelseyhightower/envconfig v1.4.0
+	github.com/nats-io/nats.go v1.31.0
+)
+
+require (
+	github.com/klauspost/compress v1.17.0 // indirect
+	github.com/nats-io/nkeys v0.4.5 // indirect
+	github.com/nats-io/nuid v1.0.1 // indirect
+	golang.org/x/crypto v0.6.0 // indirect
+	golang.org/x/sys v0.5.0 // indirect
+)