Fix gt sling --naked to bypass pane lookup for terminated polecats

When slinging to an existing polecat with --naked flag, the code was still
attempting to look up the tmux pane, which fails for terminated polecats.
Now resolveTargetAgent accepts a skipPane parameter that bypasses the tmux
pane and working directory lookup when true.

This allows work to be slung to terminated polecats that will be restarted
manually later.

Also updated unsling to skip pane lookup since it only needs the agent ID.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/cmd/unsling.go

@@ -71,7 +71,8 @@ func runUnsling(cmd *cobra.Command, args []string) error {
 	var agentID string
 	var err error
 	if targetAgent != "" {
-		agentID, _, _, err = resolveTargetAgent(targetAgent)
+		// Skip pane lookup - unsling only needs agent ID, not tmux session
+		agentID, _, _, err = resolveTargetAgent(targetAgent, true)
 		if err != nil {
 			return fmt.Errorf("resolving target agent: %w", err)
 		}