fix(witness,rig): code review cleanup

- Remove unused workDir field from witness manager
- Use witMgr.IsRunning() consistently instead of direct tmux call

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

internal/cmd/rig.go

@@ -1073,9 +1073,8 @@ func runRigStatus(cmd *cobra.Command, args []string) error {
 
 	// Witness status
 	fmt.Printf("%s\n", style.Bold.Render("Witness"))
-	witnessSession := fmt.Sprintf("gt-%s-witness", rigName)
-	witnessRunning, _ := t.HasSession(witnessSession)
-	_ = witness.NewManager(r) // silence unused warning, manager created for consistency
+	witMgr := witness.NewManager(r)
+	witnessRunning, _ := witMgr.IsRunning()
 	if witnessRunning {
 		fmt.Printf("  %s running\n", style.Success.Render("●"))
 	} else {

internal/witness/manager.go

@@ -27,15 +27,13 @@ var (
 // Manager handles witness lifecycle and monitoring operations.
 // ZFC-compliant: tmux session is the source of truth for running state.
 type Manager struct {
-	rig     *rig.Rig
-	workDir string
+	rig *rig.Rig
 }
 
 // NewManager creates a new witness manager for a rig.
 func NewManager(r *rig.Rig) *Manager {
 	return &Manager{
-		rig:     r,
-		workDir: r.Path,
+		rig: r,
 	}
 }