(subagents): ghost-rescan test gaps, deleted snapshot titles, flush pruning order (follow-up #903) #904

Follow-up of #903 (#886), review minors.

1. Untested ghost-rescan branches. remote-index.js ~310-312 (a failed scan stays eligible for retry), ~437 (checkedGhostFolders.clear() in stop()), ~202-204 (cleared on host removal): removing any of them keeps the suite green. Add one test where scanFolders returns { ok: false } and the next refresh repeats the full scan with no subset, one for restart, one for host removal.
2. A deleted run snapshot no longer notifies. main.js ~3614-3621: ENOENT falls into the same silent catch as partial JSON, so the old title stays in workflowTitles and the renderer gets no refresh while the projects query would show the run id. Treat ENOENT as "fall back to the run id" (notify if changed, delete the key); keep the silent path for parse errors. Whether the CLI ever deletes these snapshots is unverified.
3. Titles read and pruned in the same flush. main.js ~3611-3637: snapshots are read before the folder loop; a flush that also has a folder-scope event for the same folder deletes the titles just read, and the next status-only snapshot event notifies again as "first". Prune folder scope before the snapshot loop.
4. Doc indentation churn. .ai/contexts/session-cache.md ~921-925: an existing line was re-indented from 2 to 4 spaces; keep the 2-space continuation.