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/config.go

@@ -0,0 +1,38 @@
+package completions
+
+import (
+	"runtime"
+
+	"github.com/mitchellh/go-homedir"
+
+	"github.com/cheat/cheat/internal/config"
+)
+
+// loadConfig loads the cheat configuration for use in completion functions.
+// It returns an error rather than exiting, since completions should degrade
+// gracefully.
+func loadConfig() (config.Config, error) {
+	home, err := homedir.Dir()
+	if err != nil {
+		return config.Config{}, err
+	}
+
+	envvars := config.EnvVars()
+
+	confpaths, err := config.Paths(runtime.GOOS, home, envvars)
+	if err != nil {
+		return config.Config{}, err
+	}
+
+	confpath, err := config.Path(confpaths)
+	if err != nil {
+		return config.Config{}, err
+	}
+
+	conf, err := config.New(confpath, true)
+	if err != nil {
+		return config.Config{}, err
+	}
+
+	return conf, nil
+}