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>

cmd/cheat/cmd_conf.go

@@ -3,9 +3,11 @@ package main
 import (
 	"fmt"
 
+	"github.com/spf13/cobra"
+
 	"github.com/cheat/cheat/internal/config"
 )
 
-func cmdConf(_ map[string]interface{}, conf config.Config) {
+func cmdConf(_ *cobra.Command, _ []string, conf config.Config) {
 	fmt.Println(conf.Path)
 }