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.

doc/pending.txt

@@ -348,6 +348,7 @@ Default buffer-local keys: ~
     `gw`          Toggle work-in-progress status (`wip`)
     `gb`          Toggle blocked status (`blocked`)
     `g/`          Toggle cancelled status (`cancelled`)
+    `ge`          Open markdown detail buffer for task notes (`edit_notes`)
     `gf`          Prompt for filter predicates (`filter`)
     `<Tab>`       Switch between category / queue view (`view`)
     `gz`          Undo the last `:w` save (`undo`)
@@ -487,6 +488,12 @@ old keys to `false`: >lua
     Decrement the priority level for the task under the cursor, clamped
     at 0. Default key: `<C-x>`.
 
+                                               *<Plug>(pending-edit-notes)*
+<Plug>(pending-edit-notes)
+    Open the markdown detail buffer for the task under the cursor.
+    Shows a read-only metadata header and editable notes below a `---`
+    separator. Press `q` to return to the task list. Default key: `ge`.
+
                                                    *<Plug>(pending-open-line)*
 <Plug>(pending-open-line)
     Insert a correctly-formatted blank task line below the cursor.
@@ -557,6 +564,29 @@ Queue view: ~                                             *pending-view-queue*
     virtual text so tasks remain identifiable across categories. The
     buffer is named `pending://queue`.
 
+==============================================================================
+DETAIL BUFFER                                           *pending-detail-buffer*
+
+Press `ge` (or `keymaps.edit_notes`) on a task to open a markdown detail
+buffer named `pending://task/<id>`. The buffer replaces the task list in
+the same split.
+
+Layout: ~
+
+    Line 1:     `# <description>` (task description as heading)
+    Lines 2-3:  Read-only metadata (status, priority, category, due,
+                recurrence) rendered as virtual text overlays
+    Line 4:     `---` separator
+    Line 5+:    Free-form markdown notes (editable)
+
+The metadata header is not editable — it is rendered via extmarks on
+empty buffer lines. To change metadata, return to the task list and use
+the normal keymaps or `:Pending edit`.
+
+Write (`:w`) saves the notes content (everything below the `---`
+separator) to the `notes` field in the task store. Press `q` to return
+to the task list.
+
 ==============================================================================
 FILTERS                                                       *pending-filters*
 
@@ -789,6 +819,7 @@ loads: >lua
         wip = 'gw',
         blocked = 'gb',
         cancelled = 'g/',
+        edit_notes = 'ge',
       },
       sync = {
         gcal = {},
@@ -1651,6 +1682,7 @@ Task fields: ~
     {entry}       (string)  ISO 8601 UTC timestamp of creation.
     {modified}    (string)  ISO 8601 UTC timestamp of last modification.
     {end}         (string)  ISO 8601 UTC timestamp of completion or deletion.
+    {notes}       (string)  Free-form markdown notes (from detail buffer).
     {order}       (integer) Relative ordering within a category.
 
 Any field not in the list above is preserved in `_extra` and written back on