Fix gt sling code review issues (gt-z9qoo)

- Fix hook location for remote targets using tmux.GetPaneWorkDir()
- Add --var/--on conflict error check
- Add warning for JSON parse fallback (not yet fatal)
- Capture stderr for wisp command
- Remove dead code: detectAgentIdentity(), detectAgentIdentityForHook(),
  detectCloneRootForHook()
- Extract resolveSelfTarget() helper to reduce role-switching duplication

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

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

internal/cmd/handoff.go

@@ -494,18 +494,12 @@ func hookBeadForHandoff(beadID string) error {
 		return fmt.Errorf("bead '%s' not found", beadID)
 	}
 
-	// Determine agent identity
-	agentID, err := detectAgentIdentity()
+	// Determine agent identity and clone root
+	agentID, _, cloneRoot, err := resolveSelfTarget()
 	if err != nil {
 		return fmt.Errorf("detecting agent identity: %w", err)
 	}
 
-	// Get clone root for wisp storage
-	cloneRoot, err := detectCloneRoot()
-	if err != nil {
-		return fmt.Errorf("detecting clone root: %w", err)
-	}
-
 	// Create the slung work wisp
 	sw := wisp.NewSlungWork(beadID, agentID)
 	sw.Subject = handoffSubject