feat: migrate from docopt to cobra for CLI argument parsing

Replace docopt-go with spf13/cobra, giving cheat a built-in
`--completion` flag that dynamically generates shell completions for
bash, zsh, fish, and powershell. This replaces the manually-maintained
static completion scripts and resolves the root cause of multiple
completion issues (#768, #705, #633, #632, #476).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/completions/generate.go

@@ -0,0 +1,24 @@
+package completions
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/spf13/cobra"
+)
+
+// Generate writes a shell completion script to the given writer.
+func Generate(cmd *cobra.Command, shell string, w io.Writer) error {
+	switch shell {
+	case "bash":
+		return cmd.Root().GenBashCompletionV2(w, true)
+	case "zsh":
+		return cmd.Root().GenZshCompletion(w)
+	case "fish":
+		return cmd.Root().GenFishCompletion(w, true)
+	case "powershell":
+		return cmd.Root().GenPowerShellCompletionWithDesc(w)
+	default:
+		return fmt.Errorf("unsupported shell: %s (valid: bash, zsh, fish, powershell)", shell)
+	}
+}