docs: improve CLI help, shell completion, and README

- Add command groups (Getting Started, Search, Downloads, Daemon, System)
- Add shell completion command (bash, zsh, fish, powershell)
- Add flag completions for --type, --quality, --sort, --lang, --genre,
  --country, --method, --player
- Improve Long descriptions and Examples for all commands
- Split doctor disk check into platform-specific files (Unix/Windows)
- Validate infoHash length before truncating (prevent panic)
- Fix references to non-existent 'unarr daemon start' command
- Move stats command to System & Diagnostics group
- Rewrite README with complete documentation, correct config format
  (toml not yaml), all commands, shell completion section

internal/cmd/version_cmd.go

@@ -11,6 +11,7 @@ func newVersionCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "version",
 		Short: "Show unarr version",
+		Long:  "Print the unarr version, operating system, and architecture.",
 		Run: func(cmd *cobra.Command, args []string) {
 			fmt.Printf("unarr %s (%s/%s)\n", Version, runtime.GOOS, runtime.GOARCH)
 		},