Rebrand user-visible CLI text; add legacy fallbacks for keyring, cache dir and version prefix

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/clientcache/cmd/cache/stop.go

@@ -17,16 +17,16 @@ type StopCommand struct {
 }
 
 func (c *StopCommand) Synopsis() string {
-	return "Stop the Boundary cache"
+	return "Stop the Threshold cache"
 }
 
 func (c *StopCommand) Help() string {
 	helpText := `
-Usage: boundary cache stop
+Usage: threshold cache stop
 
   Stop a cache:
 
-      $ boundary cache stop
+      $ threshold cache stop
 
   For a full list of examples, please see the documentation.