feat(buffer): open as bottom-drawer split like fugitive

Problem: :Pending replaced the current buffer, making it impossible to
view tasks alongside the file being edited. No way to close the drawer
without :q or switching buffers manually.

Solution: open the task buffer in a botright horizontal split instead of
replacing the current buffer. Track the drawer window ID so re-opening
focuses it rather than creating a second split. Set winfixheight so the
drawer keeps its height when other windows open or close. Add q/<Esc>
mappings to close the drawer, and a WinClosed autocmd to clear the
tracked window ID when the user closes it manually. Add drawer_height
config option (default 15).

lua/pending/config.lua

@@ -9,6 +9,7 @@
 ---@field date_format string
 ---@field date_syntax string
 ---@field category_order? string[]
+---@field drawer_height integer
 ---@field gcal? pending.GcalConfig
 
 ---@class pending.config
@@ -22,6 +23,7 @@ local defaults = {
   date_format = '%b %d',
   date_syntax = 'due',
   category_order = {},
+  drawer_height = 15,
 }
 
 ---@type pending.Config?