feat: complete Tasks 1-3 (setup, models+storage, config management)

- Task 1: Project setup (go.mod, Makefile, .gitignore, main.go)
- Task 2: Core data models (Host, KeyPair, Snippet, AppConfig) + JSON storage layer
- Task 3: Configuration management with cross-platform path support (macOS/Linux/Windows)
- Updated PROJECT_STATE.md with progress tracking

cmd/hostkeeper/main.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var (
+	version   = "dev"
+	buildTime = "unknown"
+)
+
+func main() {
+	if len(os.Args) > 1 && os.Args[1] == "--version" {
+		fmt.Printf("hostkeeper %s (built: %s)\n", version, buildTime)
+		return
+	}
+
+	fmt.Println("HostKeeper - SSH/SFTP Management Tool")
+	fmt.Println("Run 'hostkeeper --help' for usage.")
+}