fix: refuse to git fetch while the Docker case list cannot be read (release review)

readDockerCases() reads an unreadable or half-written docker-cases.json as [], so a fetch (which runs
repository hooks, and runs unattended on the interval) could run inside a Docker workspace. The fetch
paths now read it through a strict reader: the Fetch button answers a failed run (state 'error') and the
scheduler skips the tick, undoes its picks and logs once. The status and diff routes are unchanged.

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

docs/api-reference.md

@@ -1340,7 +1340,7 @@ Lists are capped (300 files and 50 commits per repository) while the counts stay
 
 `data` is `{ state, repos, reposTruncated, repoLimit, fetchedAt }`, `repos[]` each `{ name, path, ok }` and, when it failed, `failure` (`{ code, message, stderr }`: the codes as in the clone failures, plus `NO_REMOTE`). A repository that could not be fetched keeps its row rather than being dropped. The panel shows the result under its list and then re-reads, so `behind` reflects what this fetch brought.
 
-The interval is two **synced** settings in `PUT /api/settings` (`src/git-fetch-scheduler.ts`): `gitFetchEnabled` (default `false`) and `gitFetchIntervalSeconds` (30 to 3600, default 300, the gap between runs timed from the start of a run so a slow repository does not push the next one out). Synced, not per-device, because the server is what acts on them; the scheduler reads them fresh on each 15 s tick, so a change works without a restart. A session can override both, and the override wins even while the app-wide switch is off: `POST /api/sessions/:id/git-fetch-settings` takes `{ "enabled": true | false | null, "intervalSeconds": number | null }` — `null` on a field clears that half so the app value applies again, a field left out of the body leaves the other half as it was — is `400 INVALID_INPUT` for a remote or Docker session, and answers `200` with `{ gitFetchSettings: { enabled, intervalSeconds } }` where `null` means the session follows the app. What the last run did rides the session state as `gitFetch` (`{ ranAt, trigger, state, repos, failed, firstFailure? }`, `trigger` is `click` or `interval`), on the existing `SessionUpdated` event: no new event. Sessions on the same folder share one run and both see its result, the schedule is keyed by folder, and a folder is fetched by at most one run at a time; a folder is due when ANY session open on it is due, so the order the sessions were read cannot decide which interval wins, and the folder is probed once for the whole group rather than once per session on it; every session open on a folder is remembered, so closing the one that holds the override does not stop that folder's fetching while another session is still open on it. The tick skips anything its bounded probe does not confirm is a directory, stamps each folder's schedule when it picks it, and does not run a second tick over a selection still going. Remote and Docker sessions are never fetched on a timer, and the Docker rule is re-checked against the CURRENT Docker roots every tick.
+The interval is two **synced** settings in `PUT /api/settings` (`src/git-fetch-scheduler.ts`): `gitFetchEnabled` (default `false`) and `gitFetchIntervalSeconds` (30 to 3600, default 300, the gap between runs timed from the start of a run so a slow repository does not push the next one out). Synced, not per-device, because the server is what acts on them; the scheduler reads them fresh on each 15 s tick, so a change works without a restart. A session can override both, and the override wins even while the app-wide switch is off: `POST /api/sessions/:id/git-fetch-settings` takes `{ "enabled": true | false | null, "intervalSeconds": number | null }` — `null` on a field clears that half so the app value applies again, a field left out of the body leaves the other half as it was — is `400 INVALID_INPUT` for a remote or Docker session, and answers `200` with `{ gitFetchSettings: { enabled, intervalSeconds } }` where `null` means the session follows the app. What the last run did rides the session state as `gitFetch` (`{ ranAt, trigger, state, repos, failed, firstFailure? }`, `trigger` is `click` or `interval`), on the existing `SessionUpdated` event: no new event. Sessions on the same folder share one run and both see its result, the schedule is keyed by folder, and a folder is fetched by at most one run at a time; a folder is due when ANY session open on it is due, so the order the sessions were read cannot decide which interval wins, and the folder is probed once for the whole group rather than once per session on it; every session open on a folder is remembered, so closing the one that holds the override does not stop that folder's fetching while another session is still open on it. The tick skips anything its bounded probe does not confirm is a directory, stamps each folder's schedule when it picks it, and does not run a second tick over a selection still going. Remote and Docker sessions are never fetched on a timer, and the Docker rule is re-checked against the CURRENT Docker roots every tick. Both fetch paths fail closed on a Docker case list (`docker-cases.json`) that exists but cannot be read or parsed: a click answers `200` with `state: 'error'` and publishes that as its `gitFetch`, and a tick fetches nothing and leaves its folders due for the next tick, since without the list nothing tells which folders a container can write to (the read-only status and diff routes keep reading such a list as no Docker workspaces).
 
 ## Git worktrees of a case