fix(config-backup): probe a custom folder before the retention fallback reads it, test that Save omits a displayed-only retention

docs/api-reference.md

@@ -1158,7 +1158,7 @@ Timestamped snapshots of the files that hold what a user has customized (`src/co
 
 Tracked files (a fixed list, relative to the data dir): `settings.json`, `clis.json`, `custom-model-hosts.json`, `webhook.json`, `intents.json`, `linked-cases.json`, `remote-hosts.json`, `remote-cases.json`, `docker-hosts.json`, `docker-cases.json`, `webviews.json`, `push-keys.json`, `users.json`, `.env`. Session state (`state.json`, which also holds cron jobs and respawn presets), logs, the hook secret and caches are not.
 
-Settings (synced, in `settings.json`): `configBackupEnabled` (absent = on), `configBackupDir` (absolute path, `~` expanded; empty = `<data dir>/backups/config`), `configBackupKeepCount` (2–500, default 20), `configBackupKeepDays` (0–3650, default 30; `0` = no age limit). Retention only ever deletes snapshot folders Codeman made there, and never the newest one. Each snapshot is a `0700` folder named `cfg-YYYYMMDD-HHMMSS` (server local time; `-2`, `-3`… on a same-second collision) holding the tracked files under their own names plus a `manifest.json` (`id`, `createdAt`, `reason` of `startup`/`auto`/`manual`/`pre-restore`, `appVersion`, the change-check `hash`, an `origin` id, the `keepCount` and `keepDays` in force, and a SHA-256 and size per file), all `0600`. A snapshot is never dated earlier than the instance's newest one, so a clock that read ahead once does not make every later pass write a copy. All four routes answer `503 OPERATION_FAILED` when a custom folder does not respond, and diff and restore answer `409` for a symlinked or group/world-accessible backup as well as a checksum mismatch. Only folders with that name and a valid manifest are listed, restored or pruned. Full guide: `docs/wiki/Config-Backups.md`.
+Settings (synced, in `settings.json`): `configBackupEnabled` (absent = on), `configBackupDir` (absolute path, `~` expanded; empty = `<data dir>/backups/config`), `configBackupKeepCount` (2–500, default 20), `configBackupKeepDays` (0–3650, default 30; `0` = no age limit). A key absent from `settings.json` means the retention the newest backup in the folder recorded, and the default only when none did. Retention only ever deletes snapshot folders Codeman made there, and never the newest one. Each snapshot is a `0700` folder named `cfg-YYYYMMDD-HHMMSS` (server local time; `-2`, `-3`… on a same-second collision) holding the tracked files under their own names plus a `manifest.json` (`id`, `createdAt`, `reason` of `startup`/`auto`/`manual`/`pre-restore`, `appVersion`, the change-check `hash`, an `origin` id, the `keepCount` and `keepDays` in force, and a SHA-256 and size per file), all `0600`. A snapshot is never dated earlier than the instance's newest one, so a clock that read ahead once does not make every later pass write a copy. All four routes answer `503 OPERATION_FAILED` when a custom folder does not respond, and diff and restore answer `409` for a symlinked or group/world-accessible backup as well as a checksum mismatch. Only folders with that name and a valid manifest are listed, restored or pruned. Full guide: `docs/wiki/Config-Backups.md`.
 
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