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/init.lua

@@ -401,6 +401,9 @@ function M._setup_buf_mappings(bufnr)
     open_line_above = function()
       buffer.open_line(true)
     end,
+    edit_notes = function()
+      M.open_detail()
+    end,
   }
 
   for name, fn in pairs(actions) do
@@ -888,6 +891,46 @@ function M.toggle_status(target_status)
   end
 end
 
+---@return nil
+function M.open_detail()
+  local bufnr = buffer.bufnr()
+  if not bufnr then
+    return
+  end
+  if not require_saved() then
+    return
+  end
+  local row = vim.api.nvim_win_get_cursor(0)[1]
+  local meta = buffer.meta()
+  if not meta[row] or meta[row].type ~= 'task' then
+    return
+  end
+  local id = meta[row].id
+  if not id then
+    return
+  end
+
+  local detail_bufnr = buffer.open_detail(id)
+  if not detail_bufnr then
+    return
+  end
+
+  local group = vim.api.nvim_create_augroup('PendingDetail', { clear = true })
+  vim.api.nvim_create_autocmd('BufWriteCmd', {
+    group = group,
+    buffer = detail_bufnr,
+    callback = function()
+      buffer.save_detail()
+    end,
+  })
+
+  local km = require('pending.config').get().keymaps
+  vim.keymap.set('n', km.close or 'q', function()
+    vim.api.nvim_del_augroup_by_name('PendingDetail')
+    buffer.close_detail()
+  end, { buffer = detail_bufnr })
+end
+
 ---@param direction 'up'|'down'
 ---@return nil
 function M.move_task(direction)