refactor(forge): rename auto_close to close

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/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