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/cheatsheets.go

@@ -0,0 +1,43 @@
+// Package completions provides dynamic shell completion functions and
+// completion script generation for the cheat CLI.
+package completions
+
+import (
+	"sort"
+
+	"github.com/spf13/cobra"
+
+	"github.com/cheat/cheat/internal/sheets"
+)
+
+// Cheatsheets provides completion for cheatsheet names.
+func Cheatsheets(
+	_ *cobra.Command,
+	args []string,
+	_ string,
+) ([]string, cobra.ShellCompDirective) {
+
+	if len(args) > 0 {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+
+	conf, err := loadConfig()
+	if err != nil {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+
+	cheatsheets, err := sheets.Load(conf.Cheatpaths)
+	if err != nil {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+
+	consolidated := sheets.Consolidate(cheatsheets)
+
+	names := make([]string, 0, len(consolidated))
+	for name := range consolidated {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+
+	return names, cobra.ShellCompDirectiveNoFileComp
+}