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/inconshreveable/mousetrap/trap_windows.go

@@ -0,0 +1,42 @@
+package mousetrap
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func getProcessEntry(pid int) (*syscall.ProcessEntry32, error) {
+	snapshot, err := syscall.CreateToolhelp32Snapshot(syscall.TH32CS_SNAPPROCESS, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer syscall.CloseHandle(snapshot)
+	var procEntry syscall.ProcessEntry32
+	procEntry.Size = uint32(unsafe.Sizeof(procEntry))
+	if err = syscall.Process32First(snapshot, &procEntry); err != nil {
+		return nil, err
+	}
+	for {
+		if procEntry.ProcessID == uint32(pid) {
+			return &procEntry, nil
+		}
+		err = syscall.Process32Next(snapshot, &procEntry)
+		if err != nil {
+			return nil, err
+		}
+	}
+}
+
+// StartedByExplorer returns true if the program was invoked by the user double-clicking
+// on the executable from explorer.exe
+//
+// It is conservative and returns false if any of the internal calls fail.
+// It does not guarantee that the program was run from a terminal. It only can tell you
+// whether it was launched from explorer.exe
+func StartedByExplorer() bool {
+	pe, err := getProcessEntry(syscall.Getppid())
+	if err != nil {
+		return false
+	}
+	return "explorer.exe" == syscall.UTF16ToString(pe.ExeFile[:])
+}