refactor(types): extract inline anonymous types into named classes (#110)

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/sync/oauth.lua

@@ -17,62 +17,39 @@ local BUNDLED_CLIENT_SECRET = 'PLACEHOLDER'
 ---@field expires_in? integer
 ---@field obtained_at? integer
 
----@class pending.OAuthClient
+---@class pending.OAuthClientOpts
 ---@field name string
 ---@field scope string
 ---@field port integer
 ---@field config_key string
+
+---@class pending.OAuthClient : pending.OAuthClientOpts
 local OAuthClient = {}
 OAuthClient.__index = OAuthClient
 
+local util = require('pending.sync.util')
+
 local _active_close = nil
-local _sync_in_flight = false
 
 ---@class pending.oauth
 local M = {}
 
----@param args string[]
----@param opts? table
----@return { code: integer, stdout: string, stderr: string }
-function M.system(args, opts)
-  local co = coroutine.running()
-  if not co then
-    return vim.system(args, opts or {}):wait() --[[@as { code: integer, stdout: string, stderr: string }]]
-  end
-  vim.system(args, opts or {}, function(result)
-    vim.schedule(function()
-      coroutine.resume(co, result)
-    end)
-  end)
-  return coroutine.yield() --[[@as { code: integer, stdout: string, stderr: string }]]
-end
-
----@param fn fun(): nil
-function M.async(fn)
-  coroutine.resume(coroutine.create(fn))
-end
+M.system = util.system
+M.async = util.async
 
 ---@param client pending.OAuthClient
 ---@param name string
 ---@param callback fun(access_token: string): nil
 function M.with_token(client, name, callback)
-  if _sync_in_flight then
-    require('pending.log').warn(name .. ': Sync already in progress — please wait.')
-    return
-  end
-  _sync_in_flight = true
-  M.async(function()
-    local token = client:get_access_token()
-    if not token then
-      _sync_in_flight = false
-      require('pending.log').warn(name .. ': Not authenticated — run :Pending auth.')
-      return
-    end
-    local ok, err = pcall(callback, token)
-    _sync_in_flight = false
-    if not ok then
-      require('pending.log').error(name .. ': ' .. tostring(err))
-    end
+  util.async(function()
+    util.with_guard(name, function()
+      local token = client:get_access_token()
+      if not token then
+        require('pending.log').warn(name .. ': Not authenticated — run :Pending auth.')
+        return
+      end
+      callback(token)
+    end)
   end)
 end
 
@@ -547,7 +524,7 @@ function OAuthClient:clear_tokens()
   os.remove(self:token_path())
 end
 
----@param opts { name: string, scope: string, port: integer, config_key: string }
+---@param opts pending.OAuthClientOpts
 ---@return pending.OAuthClient
 function M.new(opts)
   return setmetatable({