fix(FN-6035): address workflow projection review feedback

packages/core/src/__tests__/store-update.test.ts

@@ -1198,5 +1198,43 @@ Task with acceptance criteria
     });
   });
 
+  describe("updateTaskAtomic", () => {
+    it("serializes read-merge-write patches against the freshest task snapshot", async () => {
+      const task = await store.createTask({ description: "atomic task projection" });
+      let releaseFirst: () => void = () => {};
+      const firstCanFinish = new Promise<void>((resolve) => {
+        releaseFirst = resolve;
+      });
+      let markFirstRead: () => void = () => {};
+      const firstRead = new Promise<void>((resolve) => {
+        markFirstRead = resolve;
+      });
+
+      const first = store.updateTaskAtomic(task.id, async (current) => {
+        markFirstRead();
+        expect(current.modifiedFiles).toBeUndefined();
+        await firstCanFinish;
+        return {
+          modifiedFiles: [...new Set([...(current.modifiedFiles ?? []), "src/a.ts"])].sort(),
+          mergeDetails: { ...(current.mergeDetails ?? {}), filesChanged: 1 },
+        };
+      });
+
+      await firstRead;
+
+      const second = store.updateTaskAtomic(task.id, (current) => ({
+        modifiedFiles: [...new Set([...(current.modifiedFiles ?? []), "src/b.ts"])].sort(),
+        mergeDetails: { ...(current.mergeDetails ?? {}), insertions: 2 },
+      }));
+
+      releaseFirst();
+      await Promise.all([first, second]);
+
+      const updated = await store.getTask(task.id);
+      expect(updated.modifiedFiles).toEqual(["src/a.ts", "src/b.ts"]);
+      expect(updated.mergeDetails).toEqual({ filesChanged: 1, insertions: 2 });
+    });
+  });
+
 
 });

packages/core/src/store.ts

@@ -7445,6 +7445,23 @@ ${TASK_UPSERT_SQL_ASSIGNMENTS}
     return this.withTaskLock(id, () => this.updateTaskUnlocked(id, updates, runContext));
   }
 
+  async updateTaskAtomic(
+    id: string,
+    updater: (
+      current: Task,
+    ) => Parameters<TaskStore["updateTask"]>[1] | null | undefined | Promise<Parameters<TaskStore["updateTask"]>[1] | null | undefined>,
+    runContext?: RunMutationContext,
+  ): Promise<Task> {
+    return this.withTaskLock(id, async () => {
+      const current = await this.readTaskJson(this.taskDir(id));
+      const updates = await updater(current);
+      if (!updates || Object.values(updates).every((value) => value === undefined)) {
+        return current;
+      }
+      return this.updateTaskUnlocked(id, updates, runContext);
+    });
+  }
+
   /**
    * Merge a validated/normalized custom-field patch into the existing values.
    * `null` in the patch deletes that field's value (the delete sentinel from