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/config.lua

@@ -82,6 +82,7 @@
 ---@field priority_up_visual? string|false
 ---@field priority_down_visual? string|false
 ---@field cancelled? string|false
+---@field edit_notes? string|false
 
 ---@class pending.CategoryViewConfig
 ---@field order? string[]
@@ -163,6 +164,7 @@ local defaults = {
     wip = 'gw',
     blocked = 'gb',
     cancelled = 'g/',
+    edit_notes = 'ge',
     priority_up = '<C-a>',
     priority_down = '<C-x>',
     priority_up_visual = 'g<C-a>',