feat: add Go-based HarborForge monitor client

- collect CPU, memory, disk, swap, load, and uptime telemetry
- detect nginx and list /etc/nginx/sites-enabled entries
- send heartbeat-v2 payload with API key auth
- provide install script, config example, and systemd unit

internal/config/config.go

@@ -0,0 +1,108 @@
+package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+type Config struct {
+	BackendURL        string `json:"backendUrl"`
+	Identifier        string `json:"identifier"`
+	APIKey            string `json:"apiKey"`
+	ReportIntervalSec int    `json:"reportIntervalSec"`
+	LogLevel          string `json:"logLevel"`
+}
+
+func Load(path string) (Config, error) {
+	cfg := Config{
+		BackendURL:        getenv("HF_MONITOR_BACKEND_URL", "https://monitor.hangman-lab.top"),
+		Identifier:        getenv("HF_MONITOR_IDENTIFIER", hostnameOr("unknown-host")),
+		APIKey:            os.Getenv("HF_MONITOR_API_KEY"),
+		ReportIntervalSec: getenvInt("HF_MONITOR_REPORT_INTERVAL", 30),
+		LogLevel:          getenv("HF_MONITOR_LOG_LEVEL", "info"),
+	}
+
+	if path != "" {
+		if fileCfg, err := loadFile(path); err == nil {
+			merge(&cfg, fileCfg)
+		} else if !os.IsNotExist(err) {
+			return cfg, fmt.Errorf("load config file: %w", err)
+		}
+	}
+
+	// env always wins over file
+	cfg.BackendURL = getenv("HF_MONITOR_BACKEND_URL", cfg.BackendURL)
+	cfg.Identifier = getenv("HF_MONITOR_IDENTIFIER", cfg.Identifier)
+	if v := os.Getenv("HF_MONITOR_API_KEY"); v != "" {
+		cfg.APIKey = v
+	}
+	cfg.ReportIntervalSec = getenvInt("HF_MONITOR_REPORT_INTERVAL", cfg.ReportIntervalSec)
+	cfg.LogLevel = getenv("HF_MONITOR_LOG_LEVEL", cfg.LogLevel)
+
+	if cfg.BackendURL == "" {
+		return cfg, fmt.Errorf("backendUrl is required")
+	}
+	if cfg.Identifier == "" {
+		return cfg, fmt.Errorf("identifier is required")
+	}
+	if cfg.ReportIntervalSec <= 0 {
+		cfg.ReportIntervalSec = 30
+	}
+	return cfg, nil
+}
+
+func loadFile(path string) (Config, error) {
+	var cfg Config
+	buf, err := os.ReadFile(path)
+	if err != nil {
+		return cfg, err
+	}
+	if err := json.Unmarshal(buf, &cfg); err != nil {
+		return cfg, err
+	}
+	return cfg, nil
+}
+
+func merge(dst *Config, src Config) {
+	if src.BackendURL != "" {
+		dst.BackendURL = src.BackendURL
+	}
+	if src.Identifier != "" {
+		dst.Identifier = src.Identifier
+	}
+	if src.APIKey != "" {
+		dst.APIKey = src.APIKey
+	}
+	if src.ReportIntervalSec > 0 {
+		dst.ReportIntervalSec = src.ReportIntervalSec
+	}
+	if src.LogLevel != "" {
+		dst.LogLevel = src.LogLevel
+	}
+}
+
+func getenv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func getenvInt(key string, fallback int) int {
+	if v := os.Getenv(key); v != "" {
+		var out int
+		if _, err := fmt.Sscanf(v, "%d", &out); err == nil && out > 0 {
+			return out
+		}
+	}
+	return fallback
+}
+
+func hostnameOr(fallback string) string {
+	name, err := os.Hostname()
+	if err != nil || name == "" {
+		return fallback
+	}
+	return name
+}