feat(backend): persistent settings store + settingsFile config

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/test/settings-store.test.ts

@@ -0,0 +1,47 @@
+import { test, expect } from "vitest";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { loadSettings, saveSettings } from "../src/settings/settings-store.js";
+import { loadConfig } from "../src/config.js";
+
+function cfgWith(file: string) {
+  return loadConfig({ THT_HARNESS_DIR: "../harness", SETTINGS_FILE: file });
+}
+
+test("loadSettings returns {} when the file does not exist", () => {
+  const dir = mkdtempSync(join(tmpdir(), "tht-set-"));
+  try {
+    expect(loadSettings(cfgWith(join(dir, "settings.json")))).toEqual({});
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test("saveSettings writes the file and loadSettings reads it back", () => {
+  const dir = mkdtempSync(join(tmpdir(), "tht-set-"));
+  try {
+    const cfg = cfgWith(join(dir, "nested", "settings.json"));
+    const saved = saveSettings(cfg, { workspace: "psd", provider: "zai", model: "glm-5.2", thinking: "medium" });
+    expect(saved.model).toBe("glm-5.2");
+    expect(loadSettings(cfg)).toEqual({ workspace: "psd", provider: "zai", model: "glm-5.2", thinking: "medium" });
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test("loadSettings returns {} on corrupt JSON (no throw)", () => {
+  const dir = mkdtempSync(join(tmpdir(), "tht-set-"));
+  try {
+    const file = join(dir, "settings.json");
+    writeFileSync(file, "{ not json");
+    expect(loadSettings(cfgWith(file))).toEqual({});
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test("loadConfig sets settingsFile from SETTINGS_FILE, default data/settings.json", () => {
+  expect(loadConfig({}).settingsFile).toBe("data/settings.json");
+  expect(loadConfig({ SETTINGS_FILE: "/x/y.json" }).settingsFile).toBe("/x/y.json");
+});