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/completion.go

@@ -0,0 +1,65 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+func newCompletionCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "completion <bash|zsh|fish|powershell>",
+		Short: "Generate shell completion scripts",
+		Long: `Generate shell completion scripts for unarr.
+
+Completions allow you to press Tab to auto-complete commands, flags,
+and arguments in your terminal. Follow the instructions for your shell below.
+
+Bash:
+  # Add to ~/.bashrc for persistent completions:
+  echo 'eval "$(unarr completion bash)"' >> ~/.bashrc
+
+  # Or generate a file (recommended for system-wide):
+  unarr completion bash > /etc/bash_completion.d/unarr
+
+Zsh:
+  # Add to ~/.zshrc for persistent completions:
+  echo 'eval "$(unarr completion zsh)"' >> ~/.zshrc
+
+  # Or if you use oh-my-zsh, place in custom completions dir:
+  mkdir -p ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/completions
+  unarr completion zsh > ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/completions/_unarr
+
+Fish:
+  # Add to fish completions dir:
+  unarr completion fish > ~/.config/fish/completions/unarr.fish
+
+PowerShell:
+  # Add to your PowerShell profile:
+  unarr completion powershell >> $PROFILE`,
+		Example: `  unarr completion bash
+  unarr completion zsh
+  unarr completion fish > ~/.config/fish/completions/unarr.fish
+  eval "$(unarr completion bash)"`,
+		Args:                  cobra.ExactArgs(1),
+		DisableFlagsInUseLine: true,
+		ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+		RunE: func(cmd *cobra.Command, args []string) error {
+			switch args[0] {
+			case "bash":
+				return rootCmd.GenBashCompletionV2(os.Stdout, true)
+			case "zsh":
+				return rootCmd.GenZshCompletion(os.Stdout)
+			case "fish":
+				return rootCmd.GenFishCompletion(os.Stdout, true)
+			case "powershell":
+				return rootCmd.GenPowerShellCompletionWithDesc(os.Stdout)
+			default:
+				return fmt.Errorf("unknown shell %q: must be one of bash, zsh, fish, powershell", args[0])
+			}
+		},
+	}
+
+	return cmd
+}