fix: apply the review items for #719 at merge

- src/web/settings-writer.ts:110: export assertSettingsWritable(), the writer's own read-for-write rule run on its own
- src/web/routes/case-routes.ts:1860: an admin's rename checks settings.json before it ends any session or moves the folder, and answers 409 CONFLICT with the writer's message (placed before the session-starting check, so nothing is awaited between it and the lock)
- test/routes/case-rename-routes.test.ts:314: the torn-file pin now expects 409 CONFLICT (no stoppedSessions, like the other pre-checks)
- test/routes/case-rename-routes.test.ts:331: new test, a live session with stopSessions and a torn file: 409 CONFLICT, cleanupSession not called
- docs/api-reference.md: the Settings paragraph says an admin's rename answers this refusal as 409 CONFLICT before ending sessions
- src/hooks-config.ts:1161, src/prompt-cache-keepwarm.ts:341: comments describe the atomic writer, not an in-place rewrite

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

src/web/routes/case-routes.ts

@@ -78,7 +78,12 @@ import {
   parseBody,
   readJsonConfig,
 } from '../route-helpers.js';
-import { mutateSettingsFile, settingsWriteErrorResponse } from '../settings-writer.js';
+import {
+  assertSettingsWritable,
+  mutateSettingsFile,
+  SettingsWriteRefusedError,
+  settingsWriteErrorResponse,
+} from '../settings-writer.js';
 import { isMultiUserMode } from '../../config/multiuser.js';
 import type { AuthUser } from '../../types.js';
 import { SseEvent } from '../sse-events.js';
@@ -1850,6 +1855,17 @@ export function registerCaseRoutes(
     if (open.some((s) => !canAccessOwned(user, s.owner))) {
       return fail(403, ApiErrorCode.FORBIDDEN, "Another user's session has this case open");
     }
+    // The admin's rename writes settings.json only after the sessions are ended and the folder moved, and
+    // the writer refuses a file that is not a JSON object: check it by the writer's own rule now, so no
+    // session is ended for a rename that cannot complete. Not between the starting check and the lock below.
+    if (isAdmin(req)) {
+      try {
+        await assertSettingsWritable();
+      } catch (err) {
+        if (err instanceof SettingsWriteRefusedError) return fail(409, ApiErrorCode.CONFLICT, err.message);
+        throw err;
+      }
+    }
     // A session being created in the folder is not in ctx.sessions yet, so it could be neither listed nor
     // stopped: it would start in the old path after the move. Checked in the same tick that takes the lock
     // below (nothing is awaited in between), and the Session constructor refuses the folder from then on.

src/web/settings-writer.ts

@@ -100,6 +100,17 @@ export function mutateSettingsFile(
   return run;
 }
 
+/**
+ * Rejects with `SettingsWriteRefusedError` when `mutateSettingsFile()` would refuse a write right
+ * now: the writer's own read-for-write rule, run on its own. For a route that does something it
+ * cannot take back (case rename ends sessions) before its settings write. A check only, outside
+ * the chain (writes are atomic, so it never reads a half-written file); the file can still change
+ * before the write.
+ */
+export async function assertSettingsWritable(): Promise<void> {
+  await readSettingsForWrite(SETTINGS_PATH);
+}
+
 /** A refused write answers 409 CONFLICT with the fix; any other failure stays OPERATION_FAILED. */
 export function settingsWriteErrorResponse(err: unknown): ApiResponse<never> {
   if (err instanceof SettingsWriteRefusedError) return createErrorResponse(ApiErrorCode.CONFLICT, err.message);