feat(backend): persistent settings store + settingsFile config

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

backend/.gitignore

@@ -0,0 +1 @@
+data/settings.json

backend/src/config.ts

@@ -3,6 +3,7 @@ export interface AppConfig {
   authMode: "none" | "mock" | "oidc";
   defaults: { provider?: string; model?: string; thinking?: string };
   maxPiProcesses: number;
+  settingsFile: string;
 }
 export function loadConfig(env: Record<string, string | undefined>): AppConfig {
   return {
@@ -13,5 +14,6 @@ export function loadConfig(env: Record<string, string | undefined>): AppConfig {
     authMode: (env.AUTH_MODE as AppConfig["authMode"]) ?? "none",
     defaults: { provider: env.PI_PROVIDER, model: env.PI_MODEL, thinking: env.PI_THINKING },
     maxPiProcesses: Number(env.MAX_PI_PROCESSES ?? 4),
+    settingsFile: env.SETTINGS_FILE ?? "data/settings.json",
   };
 }

backend/src/settings/settings-store.ts

@@ -0,0 +1,29 @@
+import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { dirname } from "node:path";
+import type { AppConfig } from "../config.js";
+
+export interface Settings {
+  workspace?: string;
+  provider?: string;
+  model?: string;
+  thinking?: string;
+}
+
+/** Read settings from cfg.settingsFile. Returns {} if missing or invalid. */
+export function loadSettings(cfg: AppConfig): Settings {
+  try {
+    const raw = readFileSync(cfg.settingsFile, "utf8");
+    const parsed = JSON.parse(raw);
+    if (parsed && typeof parsed === "object") return parsed as Settings;
+    return {};
+  } catch {
+    return {};
+  }
+}
+
+/** Persist settings (pretty JSON). Creates the parent directory if needed. */
+export function saveSettings(cfg: AppConfig, s: Settings): Settings {
+  mkdirSync(dirname(cfg.settingsFile), { recursive: true });
+  writeFileSync(cfg.settingsFile, JSON.stringify(s, null, 2) + "\n", "utf8");
+  return s;
+}

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");
+});