Problem: push/sync permanently deleted remote Google Calendar events and
Google Tasks entries whenever a local task was marked deleted, done, or
de-due'd. There was no opt-out, so a misfire could silently cause
irreversible data loss on the remote side.
Solution: add a `remote_delete` boolean to the config (default `false`).
A unified flag at `sync.remote_delete` sets the base; per-backend
overrides at `sync.gcal.remote_delete` / `sync.gtasks.remote_delete`
take precedence when non-nil. When disabled, `_extra` remote IDs are
cleared silently (unlinking) so stale IDs don't accumulate.
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