feat: add markdown detail buffer for task notes

Problem: tasks only have a one-line description. There is no way to
attach extended notes, checklists, or context to a task.

Solution: add `ge` keymap to open a `pending://task/<id>` markdown
buffer that replaces the task list in the same split. The buffer shows
a read-only metadata header (status, priority, category, due,
recurrence) rendered via extmarks, a `---` separator, and editable
notes below. `:w` saves notes to a new top-level `notes` field on the
task stored in the single `tasks.json`. `q` returns to the task list.

plugin/pending.lua

@@ -407,6 +407,10 @@ vim.keymap.set('n', '<Plug>(pending-cancelled)', function()
   require('pending').toggle_status('cancelled')
 end)
 
+vim.keymap.set('n', '<Plug>(pending-edit-notes)', function()
+  require('pending').open_detail()
+end)
+
 vim.keymap.set('n', '<Plug>(pending-priority-up)', function()
   require('pending').increment_priority()
 end)