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/config/env.go

@@ -0,0 +1,20 @@
+package config
+
+import (
+	"os"
+	"runtime"
+	"strings"
+)
+
+// EnvVars reads environment variables into a map of strings.
+func EnvVars() map[string]string {
+	envvars := map[string]string{}
+	for _, e := range os.Environ() {
+		pair := strings.SplitN(e, "=", 2)
+		if runtime.GOOS == "windows" {
+			pair[0] = strings.ToUpper(pair[0])
+		}
+		envvars[pair[0]] = pair[1]
+	}
+	return envvars
+}