test(cli): kill the surviving mutants of the kernel and the runtime

cli/mutation-scopes.json

@@ -3,7 +3,7 @@
   "scopes": {
     "kernel": {
       "mutate": "src/kernel/**/*.ts",
-      "break": 71
+      "break": 95
     },
     "tools": {
       "mutate": [
@@ -38,7 +38,7 @@
     },
     "runtime": {
       "mutate": "src/runtime/**/*.ts",
-      "break": 67
+      "break": 90
     },
     "tools-claude": {
       "mutate": "src/contexts/tools/domain/profiles/claude/**/*.ts",

cli/tests/kernel/describe-error.unit.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import { describeError, errorMessage } from "../../src/kernel/describe-error.js";
+
+describe("describeError", () => {
+  it("answers the code of a filesystem failure", () => {
+    expect(describeError(Object.assign(new Error("open failed"), { code: "ENOENT" }))).toBe(
+      "ENOENT"
+    );
+  });
+
+  it("answers the message when the code is not a string", () => {
+    expect(describeError(Object.assign(new Error("open failed"), { code: 2 }))).toBe("open failed");
+  });
+
+  it("answers the message of an Error carrying no code", () => {
+    expect(describeError(new SyntaxError("Unexpected token"))).toBe("Unexpected token");
+  });
+
+  it("stringifies a thrown value that is not an Error, whatever fields it carries", () => {
+    expect(describeError({ code: "ENOENT" })).toBe("[object Object]");
+  });
+});
+
+describe("errorMessage", () => {
+  it("answers an Error's message", () => {
+    expect(errorMessage(new Error("bad json"))).toBe("bad json");
+  });
+
+  it("stringifies a thrown value that is not an Error", () => {
+    expect(errorMessage(42)).toBe("42");
+  });
+});