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/Makefile

@@ -0,0 +1,89 @@
+.PHONY: all build build-linux build-windows clean test run
+
+# Binary names
+BINARY_NAME=corrosion-companion
+BINARY_LINUX=$(BINARY_NAME)-linux-amd64
+BINARY_WINDOWS=$(BINARY_NAME)-windows-amd64.exe
+
+# Build directory
+BUILD_DIR=bin
+
+# Go parameters
+GOCMD=go
+GOBUILD=$(GOCMD) build
+GOCLEAN=$(GOCMD) clean
+GOTEST=$(GOCMD) test
+GOGET=$(GOCMD) get
+GOMOD=$(GOCMD) mod
+
+# Build flags
+LDFLAGS=-ldflags "-s -w"
+
+all: clean build
+
+build: build-linux build-windows
+
+build-linux:
+	@echo "Building for Linux (amd64)..."
+	@mkdir -p $(BUILD_DIR)
+	GOOS=linux GOARCH=amd64 $(GOBUILD) $(LDFLAGS) -o $(BUILD_DIR)/$(BINARY_LINUX) ./cmd/agent
+	@echo "Built: $(BUILD_DIR)/$(BINARY_LINUX)"
+
+build-windows:
+	@echo "Building for Windows (amd64)..."
+	@mkdir -p $(BUILD_DIR)
+	GOOS=windows GOARCH=amd64 $(GOBUILD) $(LDFLAGS) -o $(BUILD_DIR)/$(BINARY_WINDOWS) ./cmd/agent
+	@echo "Built: $(BUILD_DIR)/$(BINARY_WINDOWS)"
+
+build-local:
+	@echo "Building for current platform..."
+	@mkdir -p $(BUILD_DIR)
+	$(GOBUILD) $(LDFLAGS) -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/agent
+	@echo "Built: $(BUILD_DIR)/$(BINARY_NAME)"
+
+clean:
+	@echo "Cleaning..."
+	@$(GOCLEAN)
+	@rm -rf $(BUILD_DIR)
+
+test:
+	@echo "Running tests..."
+	@$(GOTEST) -v ./...
+
+deps:
+	@echo "Downloading dependencies..."
+	@$(GOMOD) download
+	@$(GOMOD) tidy
+
+run: build-local
+	@echo "Running agent..."
+	@./$(BUILD_DIR)/$(BINARY_NAME)
+
+# Install systemd service (Linux only)
+install-service:
+	@echo "Installing systemd service..."
+	@sudo cp $(BUILD_DIR)/$(BINARY_LINUX) /usr/local/bin/$(BINARY_NAME)
+	@sudo cp deployment/corrosion-companion.service /etc/systemd/system/
+	@sudo systemctl daemon-reload
+	@sudo systemctl enable corrosion-companion
+	@echo "Service installed. Configure /etc/corrosion-companion/.env then start with: sudo systemctl start corrosion-companion"
+
+# Development helpers
+dev: build-local
+	@./$(BUILD_DIR)/$(BINARY_NAME)
+
+fmt:
+	@echo "Formatting code..."
+	@$(GOCMD) fmt ./...
+
+lint:
+	@echo "Running linter..."
+	@golangci-lint run ./...
+
+# Show build info
+info:
+	@echo "Build Information:"
+	@echo "  Binary Name: $(BINARY_NAME)"
+	@echo "  Linux Binary: $(BUILD_DIR)/$(BINARY_LINUX)"
+	@echo "  Windows Binary: $(BUILD_DIR)/$(BINARY_WINDOWS)"
+	@echo "  Go Version: $(shell go version)"