feat(sync): add opt-in remote deletion for gcal and gtasks (#85)

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.

lua/pending/config.lua

@@ -7,16 +7,19 @@
 ---@field category string
 
 ---@class pending.GcalConfig
+---@field remote_delete? boolean
 ---@field credentials_path? string
 ---@field client_id? string
 ---@field client_secret? string
 
 ---@class pending.GtasksConfig
+---@field remote_delete? boolean
 ---@field credentials_path? string
 ---@field client_id? string
 ---@field client_secret? string
 
 ---@class pending.SyncConfig
+---@field remote_delete? boolean
 ---@field gcal? pending.GcalConfig
 ---@field gtasks? pending.GtasksConfig