refactor(forge): rename auto_close to close (#137)

Problem: `auto_close` is verbose given it's already namespaced under
`forge.` in the config table.

Solution: Rename to `close` in config defaults, class annotation,
`refresh()` usage, and vimdoc.

lua/pending/config.lua

@@ -40,7 +40,7 @@
 ---@field shorthand? string
 
 ---@class pending.ForgeConfig
----@field auto_close? boolean
+---@field close? boolean
 ---@field warn_missing_cli? boolean
 ---@field [string] pending.ForgeInstanceConfig
 
@@ -154,7 +154,7 @@ local defaults = {
   },
   sync = {},
   forge = {
-    auto_close = false,
+    close = false,
     warn_missing_cli = true,
     github = {
       icon = '',

lua/pending/forge.lua

@@ -310,7 +310,7 @@ function M.refresh(s)
           any_fetched = true
           local forge_cfg = config.get().forge or {}
           if
-            forge_cfg.auto_close
+            forge_cfg.close
             and (cache.state == 'closed' or cache.state == 'merged')
             and (task.status == 'pending' or task.status == 'wip' or task.status == 'blocked')
           then