feat(rig): support parking multiple rigs in single call

- gt rig park now accepts variadic args (fixes #375)
- gt rig unpark updated for consistency
- Errors collected and reported at end

Also fixes test self-interruption bug where sling tests sent real
tmux nudges containing "Work slung: gt-wisp-xyz", causing agents
running tests to interrupt themselves. Added GT_TEST_NO_NUDGE env
var to skip nudge during tests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/sling_helpers.go

@@ -175,6 +175,11 @@ func injectStartPrompt(pane, beadID, subject, args string) error {
 		return fmt.Errorf("no target pane")
 	}
 
+	// Skip nudge during tests to prevent agent self-interruption
+	if os.Getenv("GT_TEST_NO_NUDGE") != "" {
+		return nil
+	}
+
 	// Build the prompt to inject
 	var prompt string
 	if args != "" {