Fix merge-ready task supersession handling

src/db/paired-turns.ts

@@ -463,6 +463,20 @@ export function failPairedTurnInDatabase(
   });
 }
 
+export function cancelPairedTurnInDatabase(
+  database: Database,
+  args: {
+    turnIdentity: PairedTurnIdentity;
+    error?: string | null;
+  },
+): void {
+  markPairedTurnTerminalStateInDatabase(database, {
+    turnIdentity: args.turnIdentity,
+    state: 'cancelled',
+    error: args.error,
+  });
+}
+
 export function getPairedTurnByIdFromDatabase(
   database: Database,
   turnId: string,

src/db/runtime-paired.ts

@@ -50,6 +50,7 @@ import {
   insertPairedTurnOutputInDatabase,
 } from './paired-turn-outputs.js';
 import {
+  cancelPairedTurnInDatabase,
   clearPairedTurnsInDatabase,
   completePairedTurnInDatabase,
   failPairedTurnInDatabase,
@@ -176,6 +177,13 @@ export function failPairedTurn(args: {
   failPairedTurnInDatabase(requireDatabase(), args);
 }
 
+export function cancelPairedTurn(args: {
+  turnIdentity: PairedTurnIdentity;
+  error?: string | null;
+}): void {
+  cancelPairedTurnInDatabase(requireDatabase(), args);
+}
+
 export function getPairedTurnById(
   turnId: string,
 ): PairedTurnRecord | undefined {