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>

docs/api-reference.md

@@ -1213,7 +1213,7 @@ A local case whose folder is the top of a git repository can keep **worktrees**
 
 ## Settings
 
-`~/.codeman/settings.json` is one file for the whole server: the synced App Settings (feature switches, the plan-usage collection switch `showPlanUsageLimits`, `runMode`, `respawnPresets`, `tunnelEnabled`, ...), plus `modelConfig`, `caseOrder` and `lastUsedCase`. Reads are open to every signed-in user; in multi-user mode a non-admin reads it without what is the admin's (`voiceSettings.apiKey`, the Deepgram key an admin saved, and the admin's case names in `caseOrder` and `lastUsedCase`). **In multi-user mode every write is admin only:** for a non-admin the routes below answer `403 FORBIDDEN` before the body is even validated, so nothing is written, no watcher is toggled and no tunnel is started. Single-user mode is unaffected. Every write to the file (these routes, and the `lastUsedCase` that `POST /api/quick-start` and `POST /api/ralph-loop/start` record before they answer) runs one at a time and replaces the file atomically (mode `0600`); the one write outside that queue is a config-backup restore, which must work on a file that does not parse. A write to a `settings.json` that exists but is not a JSON object is refused rather than starting from `{}`, which would erase every other setting: the routes below answer `409 CONFLICT` with a message naming the fix and leave the file untouched, the two starts skip their `lastUsedCase`, and `POST /api/cases/:name/rename` fails and is undone like any other registry write failure. A missing or empty file still starts from `{}`.
+`~/.codeman/settings.json` is one file for the whole server: the synced App Settings (feature switches, the plan-usage collection switch `showPlanUsageLimits`, `runMode`, `respawnPresets`, `tunnelEnabled`, ...), plus `modelConfig`, `caseOrder` and `lastUsedCase`. Reads are open to every signed-in user; in multi-user mode a non-admin reads it without what is the admin's (`voiceSettings.apiKey`, the Deepgram key an admin saved, and the admin's case names in `caseOrder` and `lastUsedCase`). **In multi-user mode every write is admin only:** for a non-admin the routes below answer `403 FORBIDDEN` before the body is even validated, so nothing is written, no watcher is toggled and no tunnel is started. Single-user mode is unaffected. Every write to the file (these routes, and the `lastUsedCase` that `POST /api/quick-start` and `POST /api/ralph-loop/start` record before they answer) runs one at a time and replaces the file atomically (mode `0600`); the one write outside that queue is a config-backup restore, which must work on a file that does not parse. A write to a `settings.json` that exists but is not a JSON object is refused rather than starting from `{}`, which would erase every other setting: the routes below answer `409 CONFLICT` with a message naming the fix and leave the file untouched, the two starts skip their `lastUsedCase`, and an admin's `POST /api/cases/:name/rename` answers the same `409 CONFLICT`, checked before it ends any session or moves the folder. A missing or empty file still starts from `{}`.
 
 | Method | Path                          | Body                                 | Notes |
 | ------ | ----------------------------- | ------------------------------------ | ----- |