refactor(types): extract inline anonymous types into named classes

Problem: several functions used inline `{...}` table types in their
`@param` and `@return` annotations, making them hard to read and
impossible to reference from other modules.

Solution: extract each into a named `---@class`: `pending.Metadata`,
`pending.TaskFields`, `pending.CompletionItem`, `pending.SystemResult`,
and `pending.OAuthClientOpts`.

lua/pending/store.lua

@@ -22,6 +22,17 @@ local config = require('pending.config')
 ---@field undo pending.Task[][]
 ---@field folded_categories string[]
 
+---@class pending.TaskFields
+---@field description string
+---@field status? string
+---@field category? string
+---@field priority? integer
+---@field due? string
+---@field recur? string
+---@field recur_mode? string
+---@field order? integer
+---@field _extra? table
+
 ---@class pending.Store
 ---@field path string
 ---@field _data pending.Data?
@@ -264,7 +275,7 @@ function Store:get(id)
   return nil
 end
 
----@param fields { description: string, status?: string, category?: string, priority?: integer, due?: string, recur?: string, recur_mode?: string, order?: integer, _extra?: table }
+---@param fields pending.TaskFields
 ---@return pending.Task
 function Store:add(fields)
   local data = self:data()