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/color.go

@@ -7,7 +7,7 @@ import (
 )
 
 // Color indicates whether colorization should be applied to the output
-func (c *Config) Color(opts map[string]interface{}) bool {
+func (c *Config) Color(forceColorize bool) bool {
 
 	// default to the colorization specified in the configs...
 	colorize := c.Colorize
@@ -18,7 +18,7 @@ func (c *Config) Color(opts map[string]interface{}) bool {
 	}
 
 	// ... *unless* the --colorize flag was passed
-	if opts["--colorize"] == true {
+	if forceColorize {
 		colorize = true
 	}
 

internal/config/color_test.go

@@ -10,13 +10,11 @@ func TestColor(t *testing.T) {
 	// mock a config
 	conf := Config{}
 
-	opts := map[string]interface{}{"--colorize": false}
-	if conf.Color(opts) {
-		t.Errorf("failed to respect --colorize (false)")
+	if conf.Color(false) {
+		t.Errorf("failed to respect forceColorize (false)")
 	}
 
-	opts = map[string]interface{}{"--colorize": true}
-	if !conf.Color(opts) {
-		t.Errorf("failed to respect --colorize (true)")
+	if !conf.Color(true) {
+		t.Errorf("failed to respect forceColorize (true)")
 	}
 }

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
+}