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/doctor_unix.go

@@ -0,0 +1,22 @@
+//go:build !windows
+
+package cmd
+
+import (
+	"fmt"
+	"syscall"
+)
+
+func checkDiskSpace(dir string) (string, error) {
+	var stat syscall.Statfs_t
+	if err := syscall.Statfs(dir, &stat); err != nil {
+		return "", err
+	}
+	available := int64(stat.Bavail) * int64(stat.Bsize)
+	gb := float64(available) / (1024 * 1024 * 1024)
+	msg := fmt.Sprintf("%.1f GB free", gb)
+	if gb < 10 {
+		return "!" + msg + " (low)", nil
+	}
+	return msg, nil
+}