Merge pull request #719: fix(settings): serialize settings.json writes so a quick-start cannot tear the file

Conflict in src/web/routes/system-routes.ts (PUT /api/settings) resolved by
keeping both sides: #718's line that never stores an unauthorized
tunnelEnabled:true runs first, then #719's serialized mutateSettingsFile()
write replaces the inline read and plain writeFile.

test/routes/system-routes-tunnel-guard.test.ts: #718's new tests asserted
plain writeFile calls on settings.json. With #719 every write goes to a temp
file renamed over it, so the assertions read writes through a settingsWrites()
helper (writeFile contents keyed by their rename), and the concurrent-stop
harness applies a write and the stop at the rename.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/web/server.ts

@@ -174,6 +174,7 @@ import { MAX_PASTE_IMAGE_BYTES } from '../config/buffer-limits.js';
 import { resolveTerminalHistoryConfig } from '../config/terminal-history.js';
 import { SseEvent } from './sse-events.js';
 import { MAX_ROUTE_PARAM_LENGTH } from './route-helpers.js';
+import { mutateSettingsFile } from './settings-writer.js';
 import { getLatestPlanUsage, setLatestCodexPlanUsage, setLatestCopilotPlanUsage } from './plan-usage-latest.js';
 import { telemetrySignature } from '../usage-telemetry.js';
 import { readCodexPlanUsage, resolveCodexBinaryPath } from '../utils/codex-cli-resolver.js';
@@ -3351,16 +3352,12 @@ export class WebServer extends EventEmitter {
     // Tunnel only starts when user clicks the toggle in the UI — never on boot.
     // Reset persisted tunnelEnabled so the UI toggle reflects actual state.
     if (await this.isTunnelEnabled()) {
-      const settingsPath = dataPath('settings.json');
       try {
-        const content = await fs.readFile(settingsPath, 'utf-8');
-        const settings = JSON.parse(content);
-        settings.tunnelEnabled = false;
-        await fs.writeFile(settingsPath, JSON.stringify(settings, null, 2));
-      } catch {
-        /* ignore */
+        await mutateSettingsFile((settings) => ({ ...settings, tunnelEnabled: false }));
+        console.log('Cloudflare tunnel setting reset (tunnel only starts on explicit UI toggle)');
+      } catch (err) {
+        console.warn('Could not reset the Cloudflare tunnel setting:', getErrorMessage(err));
       }
-      console.log('Cloudflare tunnel setting reset (tunnel only starts on explicit UI toggle)');
     }
 
     // Start team watcher for agent team awareness (always on — lightweight polling)