feat(KB-025): add per-task model overrides for executor and validator

- Add model override fields to Task type and TaskStore (modelProvider, modelId, validatorModelProvider, validatorModelId)

- Extend PATCH /api/tasks/:id endpoint with validation for model fields

- Create ModelSelectorTab component with provider/model dropdowns and tests

- Integrate Model tab into TaskDetailModal for per-task model configuration

- Update executor to use per-task model overrides when both provider and modelId are set

- Update reviewer to use per-task validator model overrides in reviewStep

- Document per-task model selection feature in AGENTS.md settings section

packages/core/src/store.test.ts

@@ -615,6 +615,64 @@ describe("TaskStore", () => {
     });
   });
 
+  describe("updateTask — model overrides", () => {
+    it("sets executor model provider and id via updateTask", async () => {
+      const task = await createTestTask();
+      const updated = await store.updateTask(task.id, { modelProvider: "anthropic", modelId: "claude-sonnet-4-5" });
+      expect(updated.modelProvider).toBe("anthropic");
+      expect(updated.modelId).toBe("claude-sonnet-4-5");
+    });
+
+    it("sets validator model provider and id via updateTask", async () => {
+      const task = await createTestTask();
+      const updated = await store.updateTask(task.id, { validatorModelProvider: "openai", validatorModelId: "gpt-4o" });
+      expect(updated.validatorModelProvider).toBe("openai");
+      expect(updated.validatorModelId).toBe("gpt-4o");
+    });
+
+    it("clears executor model fields via null", async () => {
+      const task = await createTestTask();
+      await store.updateTask(task.id, { modelProvider: "anthropic", modelId: "claude-sonnet-4-5" });
+      const updated = await store.updateTask(task.id, { modelProvider: null, modelId: null });
+      expect(updated.modelProvider).toBeUndefined();
+      expect(updated.modelId).toBeUndefined();
+    });
+
+    it("clears validator model fields via null", async () => {
+      const task = await createTestTask();
+      await store.updateTask(task.id, { validatorModelProvider: "openai", validatorModelId: "gpt-4o" });
+      const updated = await store.updateTask(task.id, { validatorModelProvider: null, validatorModelId: null });
+      expect(updated.validatorModelProvider).toBeUndefined();
+      expect(updated.validatorModelId).toBeUndefined();
+    });
+
+    it("sets only executor model without affecting validator model", async () => {
+      const task = await createTestTask();
+      await store.updateTask(task.id, { validatorModelProvider: "openai", validatorModelId: "gpt-4o" });
+      const updated = await store.updateTask(task.id, { modelProvider: "anthropic", modelId: "claude-sonnet-4-5" });
+      expect(updated.modelProvider).toBe("anthropic");
+      expect(updated.modelId).toBe("claude-sonnet-4-5");
+      expect(updated.validatorModelProvider).toBe("openai");
+      expect(updated.validatorModelId).toBe("gpt-4o");
+    });
+
+    it("preserves model fields when updating unrelated fields", async () => {
+      const task = await createTestTask();
+      await store.updateTask(task.id, {
+        modelProvider: "anthropic",
+        modelId: "claude-sonnet-4-5",
+        validatorModelProvider: "openai",
+        validatorModelId: "gpt-4o",
+      });
+      const updated = await store.updateTask(task.id, { title: "Updated title" });
+      expect(updated.modelProvider).toBe("anthropic");
+      expect(updated.modelId).toBe("claude-sonnet-4-5");
+      expect(updated.validatorModelProvider).toBe("openai");
+      expect(updated.validatorModelId).toBe("gpt-4o");
+      expect(updated.title).toBe("Updated title");
+    });
+  });
+
   describe("agent log persistence", () => {
     it("appendAgentLog creates agent.log and getAgentLogs reads it back", async () => {
       const task = await createTestTask();

packages/core/src/store.ts

@@ -340,7 +340,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
 
   async updateTask(
     id: string,
-    updates: { title?: string; description?: string; prompt?: string; worktree?: string; status?: string | null; dependencies?: string[]; blockedBy?: string | null; paused?: boolean; baseBranch?: string; size?: "S" | "M" | "L"; reviewLevel?: number },
+    updates: { title?: string; description?: string; prompt?: string; worktree?: string; status?: string | null; dependencies?: string[]; blockedBy?: string | null; paused?: boolean; baseBranch?: string; size?: "S" | "M" | "L"; reviewLevel?: number; modelProvider?: string | null; modelId?: string | null; validatorModelProvider?: string | null; validatorModelId?: string | null },
   ): Promise<Task> {
     return this.withTaskLock(id, async () => {
       const dir = this.taskDir(id);
@@ -382,6 +382,26 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       if (updates.baseBranch !== undefined) task.baseBranch = updates.baseBranch;
       if (updates.size !== undefined) task.size = updates.size;
       if (updates.reviewLevel !== undefined) task.reviewLevel = updates.reviewLevel;
+      if (updates.modelProvider === null) {
+        task.modelProvider = undefined;
+      } else if (updates.modelProvider !== undefined) {
+        task.modelProvider = updates.modelProvider;
+      }
+      if (updates.modelId === null) {
+        task.modelId = undefined;
+      } else if (updates.modelId !== undefined) {
+        task.modelId = updates.modelId;
+      }
+      if (updates.validatorModelProvider === null) {
+        task.validatorModelProvider = undefined;
+      } else if (updates.validatorModelProvider !== undefined) {
+        task.validatorModelProvider = updates.validatorModelProvider;
+      }
+      if (updates.validatorModelId === null) {
+        task.validatorModelId = undefined;
+      } else if (updates.validatorModelId !== undefined) {
+        task.validatorModelId = updates.validatorModelId;
+      }
       task.updatedAt = new Date().toISOString();
 
       await this.atomicWriteTaskJson(dir, task);

packages/core/src/types.ts

@@ -100,6 +100,22 @@ export interface Task {
   log: TaskLogEntry[];
   size?: "S" | "M" | "L";
   reviewLevel?: number;
+  /** AI model provider override for the executor agent (e.g., "anthropic").
+   *  Must be set together with `modelId`. When both model fields are undefined,
+   *  the executor uses global settings defaults. */
+  modelProvider?: string;
+  /** AI model ID override for the executor agent (e.g., "claude-sonnet-4-5").
+   *  Must be set together with `modelProvider`. When both model fields are undefined,
+   *  the executor uses global settings defaults. */
+  modelId?: string;
+  /** AI model provider override for the validator/reviewer agent.
+   *  Must be set together with `validatorModelId`. When both validator model fields
+   *  are undefined, the reviewer uses global settings defaults. */
+  validatorModelProvider?: string;
+  /** AI model ID override for the validator/reviewer agent.
+   *  Must be set together with `validatorModelProvider`. When both validator model
+   *  fields are undefined, the reviewer uses global settings defaults. */
+  validatorModelId?: string;
   /** Number of merge retry attempts made for this task (auto-merge conflict recovery) */
   mergeRetries?: number;
   /** ISO-8601 timestamp of when the task last entered its current column.