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>

vendor/github.com/spf13/pflag/.golangci.yaml

@@ -0,0 +1,4 @@
+linters:
+    disable-all: true
+    enable:
+        - nolintlint