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.

lua/pending/views.lua

@@ -341,6 +341,7 @@ function M.priority_view(tasks)
       forge_ref = task._extra and task._extra._forge_ref or nil,
       forge_cache = task._extra and task._extra._forge_cache or nil,
       forge_spans = compute_forge_spans(task, prefix_len),
+      has_notes = task.notes ~= nil and task.notes ~= '',
     })
   end