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
 	}