feat: Implement CLI framework with Cobra

- Add root command with Cobra framework
- Implement shell completion support for bash/zsh/fish/powershell
- Add version command
- Add configuration initialization on startup via PersistentPreRunE
- Include verbose (-v/-vv) and debug flags
- Set up proper error handling and exit codes

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

cmd/hostkeeper/main.go

@@ -11,11 +11,8 @@ var (
 )
 
 func main() {
-	if len(os.Args) > 1 && os.Args[1] == "--version" {
-		fmt.Printf("hostkeeper %s (built: %s)\n", version, buildTime)
-		return
+	if err := Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
 	}
-
-	fmt.Println("HostKeeper - SSH/SFTP Management Tool")
-	fmt.Println("Run 'hostkeeper --help' for usage.")
 }