fix(delegate-task): reject stray backend-style categories

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/delegate-task/category-resolver-unknown-category.test.ts

@@ -0,0 +1,43 @@
+declare const require: (name: string) => any
+const { afterEach, beforeEach, describe, expect, mock, spyOn, test } = require("bun:test")
+import { resolveCategoryExecution } from "./category-resolver"
+import type { ExecutorContext } from "./executor-types"
+import * as availableModels from "./available-models"
+
+describe("resolveCategoryExecution unknown category handling", () => {
+	beforeEach(() => {
+		mock.restore()
+	})
+
+	afterEach(() => {
+		mock.restore()
+	})
+
+	test("#given unknown category #when resolving category execution #then it rejects before fetching available models", async () => {
+		//#given
+		const availableModelsSpy = spyOn(availableModels, "getAvailableModelsForDelegateTask")
+		const executorContext: ExecutorContext = {
+			client: {} as ExecutorContext["client"],
+			manager: {} as ExecutorContext["manager"],
+			directory: "/tmp/test",
+			userCategories: {},
+			sisyphusJuniorModel: undefined,
+		}
+		const args = {
+			category: "backend-engineer",
+			prompt: "test prompt",
+			description: "Test task",
+			run_in_background: false,
+			load_skills: [],
+			blockedBy: undefined,
+			enableSkillTools: false,
+		}
+
+		//#when
+		const result = await resolveCategoryExecution(args, executorContext, undefined, "anthropic/claude-sonnet-4-6")
+
+		//#then
+		expect(result.error).toContain('Unknown category: "backend-engineer"')
+		expect(availableModelsSpy).not.toHaveBeenCalled()
+	})
+})