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/cobra/MAINTAINERS

@@ -0,0 +1,13 @@
+maintainers:
+- spf13
+- johnSchnake
+- jpmcb
+- marckhouzam
+inactive:
+- anthonyfok
+- bep
+- bogem
+- broady
+- eparis
+- jharshman
+- wfernandes