refactor(cli): promote sync backends to top-level subcommands

Problem: `:Pending sync gtasks auth` required an extra `sync` keyword
that added no value and made the command unnecessarily verbose.

Solution: route `gtasks` and `gcal` as top-level `:Pending` subcommands
via `SYNC_BACKEND_SET` lookup. Tab completion introspects backend
modules for available actions instead of hardcoding `{ 'auth', 'sync' }`.

lua/pending/init.lua

@@ -523,14 +523,19 @@ function M.add(text)
   vim.notify('Pending added: ' .. description)
 end
 
+---@type string[]
+local SYNC_BACKENDS = { 'gcal', 'gtasks' }
+
+---@type table<string, true>
+local SYNC_BACKEND_SET = {}
+for _, b in ipairs(SYNC_BACKENDS) do
+  SYNC_BACKEND_SET[b] = true
+end
+
 ---@param backend_name string
 ---@param action? string
 ---@return nil
-function M.sync(backend_name, action)
-  if not backend_name or backend_name == '' then
-    vim.notify('Usage: :Pending sync <backend> [action]', vim.log.levels.ERROR)
-    return
-  end
+local function run_sync(backend_name, action)
   action = (action and action ~= '') and action or 'sync'
   local ok, backend = pcall(require, 'pending.sync.' .. backend_name)
   if not ok then
@@ -835,9 +840,9 @@ function M.command(args)
   elseif cmd == 'edit' then
     local id_str, edit_rest = rest:match('^(%S+)%s*(.*)')
     M.edit(id_str, edit_rest)
-  elseif cmd == 'sync' then
-    local backend, action = rest:match('^(%S+)%s*(.*)')
-    M.sync(backend, action)
+  elseif SYNC_BACKEND_SET[cmd] then
+    local action = rest:match('^(%S+)') or 'sync'
+    run_sync(cmd, action)
   elseif cmd == 'archive' then
     local d = rest ~= '' and tonumber(rest) or nil
     M.archive(d)
@@ -854,4 +859,14 @@ function M.command(args)
   end
 end
 
+---@return string[]
+function M.sync_backends()
+  return SYNC_BACKENDS
+end
+
+---@return table<string, true>
+function M.sync_backend_set()
+  return SYNC_BACKEND_SET
+end
+
 return M