feat: add markdown detail buffer for task notes (#162)

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/store.lua

@@ -24,6 +24,7 @@ local config = require('pending.config')
 ---@field entry string
 ---@field modified string
 ---@field end? string
+---@field notes? string
 ---@field order integer
 ---@field _extra? pending.TaskExtra
 
@@ -93,6 +94,7 @@ local known_fields = {
   entry = true,
   modified = true,
   ['end'] = true,
+  notes = true,
   order = true,
 }
 
@@ -124,6 +126,9 @@ local function task_to_table(task)
   if task['end'] then
     t['end'] = task['end']
   end
+  if task.notes then
+    t.notes = task.notes
+  end
   if task.order and task.order ~= 0 then
     t.order = task.order
   end
@@ -150,6 +155,7 @@ local function table_to_task(t)
     entry = t.entry,
     modified = t.modified,
     ['end'] = t['end'],
+    notes = t.notes,
     order = t.order or 0,
     _extra = {},
   }