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/stats.go

@@ -13,10 +13,14 @@ import (
 
 func newStatsCmd() *cobra.Command {
 	cmd := &cobra.Command{
-		Use:     "stats",
-		Short:   "Show system statistics",
-		Long:    "Display aggregator statistics including content counts, torrent sources, and recent ingestion history.",
-		Example: `  unarr stats`,
+		Use:   "stats",
+		Short: "Show catalog statistics",
+		Long: `Display aggregator statistics from the unarr catalog.
+
+Shows total content count, torrent count, sources breakdown, and recent
+ingestion activity. Useful for understanding the catalog coverage.`,
+		Example: `  unarr stats
+  unarr stats --json`,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			client := getClient()