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/config.go

@@ -14,11 +14,20 @@ import (
 func newConfigCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "config",
-		Short: "Configure unarr",
-		Long: `Interactive setup for unarr.
+		Short: "Edit configuration interactively",
+		Long: `Edit unarr settings interactively in your terminal.
 
-Configures the API URL, API key, default country, and saves to config file.`,
-		Example: `  unarr config`,
+Prompts for API URL, API key, and default country. Press Enter to keep
+the current value. For first-time setup use 'unarr setup' instead.
+
+Config file: ~/.config/unarr/config.toml
+Environment variables override config file values:
+  UNARR_API_KEY        API key
+  UNARR_API_URL        API URL
+  UNARR_COUNTRY        Default country code
+  UNARR_DOWNLOAD_DIR   Download directory`,
+		Example: `  unarr config
+  unarr config --config /path/to/config.toml`,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runConfig()
 		},