fix(buffer): default to natural split height like fugitive

Problem: hardcoded drawer_height=15 was too small and diverged from
fugitive's model. Fugitive issues a plain botright split and lets Vim's
own split rules (equalalways, winheight) divide the available space.

Solution: remove the default height so the split sizes naturally. Only
call nvim_win_set_height when the user sets drawer_height to a positive
value, preserving the opt-in customization path.

lua/pending/config.lua

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