fix(sling): remove obsolete --naked flag

The --naked flag (skip tmux session creation) was a vestige of an earlier
design requiring manual session management. With the current polecat
architecture where polecats are witness-managed, ephemeral, and self-deleting
after task completion, manual session management is no longer needed.

The flag also created invalid states (e.g., slinging to crew --naked left
them unreachable since crew require tmux sessions for communication).

Closes gt-xhn5s

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

internal/cmd/unsling.go

@@ -75,8 +75,7 @@ func runUnsling(cmd *cobra.Command, args []string) error {
 	var agentID string
 	var err error
 	if targetAgent != "" {
-		// Skip pane lookup - unsling only needs agent ID, not tmux session
-		agentID, _, _, err = resolveTargetAgent(targetAgent, true)
+		agentID, _, _, err = resolveTargetAgent(targetAgent)
 		if err != nil {
 			return fmt.Errorf("resolving target agent: %w", err)
 		}