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/buffer.lua

@@ -284,7 +284,10 @@ function M.open()
   vim.cmd('botright new')
   task_winid = vim.api.nvim_get_current_win()
   vim.api.nvim_win_set_buf(task_winid, task_bufnr)
-  vim.api.nvim_win_set_height(task_winid, config.get().drawer_height)
+  local h = config.get().drawer_height
+  if h and h > 0 then
+    vim.api.nvim_win_set_height(task_winid, h)
+  end
   set_win_options(task_winid)
 
   M.render(task_bufnr)

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?