fix(rig): check witness/state.json instead of witness/rig (gm-2ej)

Witnesses don't have git clones like other agents. The detection was
checking for witness/rig which doesn't exist. Now correctly checks for
witness/state.json which is created by AddRig.

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

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

internal/rig/manager_test.go

@@ -37,6 +37,12 @@ func createTestRig(t *testing.T, root, name string) {
 		}
 	}
 
+	// Create witness state.json (witnesses don't have clones, just state)
+	witnessState := filepath.Join(rigPath, "witness", "state.json")
+	if err := os.WriteFile(witnessState, []byte(`{"role":"witness"}`), 0644); err != nil {
+		t.Fatalf("write witness state: %v", err)
+	}
+
 	// Create some polecats
 	polecatsDir := filepath.Join(rigPath, "polecats")
 	for _, polecat := range []string{"Toast", "Cheedo"} {