feat(buffer): open as bottom-drawer split like fugitive (#23)

* 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).

* 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/init.lua

@@ -38,11 +38,25 @@ function M._setup_autocmds(bufnr)
       end
     end,
   })
+  vim.api.nvim_create_autocmd('WinClosed', {
+    group = group,
+    callback = function(ev)
+      if tonumber(ev.match) == buffer.winid() then
+        buffer.clear_winid()
+      end
+    end,
+  })
 end
 
 ---@param bufnr integer
 function M._setup_buf_mappings(bufnr)
   local opts = { buffer = bufnr, silent = true }
+  vim.keymap.set('n', 'q', function()
+    buffer.close()
+  end, opts)
+  vim.keymap.set('n', '<Esc>', function()
+    buffer.close()
+  end, opts)
   vim.keymap.set('n', '<CR>', function()
     M.toggle_complete()
   end, opts)