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/Makefile

@@ -0,0 +1,35 @@
+BIN="./bin"
+SRC=$(shell find . -name "*.go")
+
+ifeq (, $(shell which golangci-lint))
+$(warning "could not find golangci-lint in $(PATH), run: curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh")
+endif
+
+.PHONY: fmt lint test install_deps clean
+
+default: all
+
+all: fmt test
+
+fmt:
+	$(info ******************** checking formatting ********************)
+	@test -z $(shell gofmt -l $(SRC)) || (gofmt -d $(SRC); exit 1)
+
+lint:
+	$(info ******************** running lint tools ********************)
+	golangci-lint run -v
+
+test: install_deps
+	$(info ******************** running tests ********************)
+	go test -v ./...
+
+richtest: install_deps
+	$(info ******************** running tests with kyoh86/richgo ********************)
+	richgo test -v ./...
+
+install_deps:
+	$(info ******************** downloading dependencies ********************)
+	go get -v ./...
+
+clean:
+	rm -rf $(BIN)