refactor(views): adopt markdown checkbox line format

Problem: task lines used an opaque /ID/  [N] prefix format that was
hard to read and inconsistent between category and priority views.
Header lines had no visual marker distinguishing them from tasks.

Solution: render headers as '## Cat', task lines as
'/ID/- [x|!| ] description'. State encoding: [x]=done, [!]=urgent,
[ ]=pending. Both views use the same construction.

lua/pending/views.lua

@@ -125,7 +125,7 @@ function M.category_view(tasks)
       table.insert(lines, '')
       table.insert(meta, { type = 'blank' })
     end
-    table.insert(lines, cat)
+    table.insert(lines, '## ' .. cat)
     table.insert(meta, { type = 'header', category = cat })
 
     local all = {}
@@ -138,9 +138,8 @@ function M.category_view(tasks)
 
     for _, task in ipairs(all) do
       local prefix = '/' .. task.id .. '/'
-      local indent = '  '
-      local prio = task.priority > 0 and ('[' .. task.priority .. '] ') or ''
-      local line = prefix .. indent .. prio .. task.description
+      local state = task.status == 'done' and 'x' or (task.priority > 0 and '!' or ' ')
+      local line = prefix .. '- [' .. state .. '] ' .. task.description
       table.insert(lines, line)
       table.insert(meta, {
         type = 'task',
@@ -189,9 +188,8 @@ function M.priority_view(tasks)
 
   for _, task in ipairs(all) do
     local prefix = '/' .. task.id .. '/'
-    local indent = '  '
-    local prio = task.priority == 1 and '! ' or ''
-    local line = prefix .. indent .. prio .. task.description
+    local state = task.status == 'done' and 'x' or (task.priority > 0 and '!' or ' ')
+    local line = prefix .. '- [' .. state .. '] ' .. task.description
     table.insert(lines, line)
     table.insert(meta, {
       type = 'task',