refactor: adopt markdown-style checkbox buffer format (#20)

* refactor(config): change default category from Inbox to Todo

* 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.

* refactor(diff): parse and reconcile markdown checkbox format

Problem: parse_buffer matched the old '  text' indent pattern and
detected headers via '^%S'. Priority was read from a '[N] ' prefix.
apply() never reconciled status changes written into the buffer.

Solution: match '- [.] text' for tasks and '^## ' for headers.
Extract state char to derive priority (! -> 1) and status (x -> done).
apply() now reconciles status from the buffer, setting/clearing 'end'
timestamps — enabling the oil-style edit-checkbox-then-:w workflow.

* refactor(buffer): update syntax, extmarks, and render for checkbox format

Problem: syntax patterns matched the old indent/[N] format; right_align
virtual text produced a broken layout in narrow windows; the done
strikethrough skipped past the '  ' indent leaving '- [x] ' unstyled;
render() added undo history entries so 'u' could undo a re-render.

Solution: update taskHeader/taskLine patterns for '## '/'- [.]'; rename
taskPriority -> taskCheckbox matching '[!]'; switch virt_text_pos to
'eol'; drop the +2 col_start offset so strikethrough covers '- [x] ';
guard nvim_buf_set_lines with undolevels=-1 so renders are not undoable.
Also fix open_line to insert '- [ ] ' and position cursor at col 6.

* refactor(init): replace multi-level priority with binary toggle

Problem: <C-a>/<C-x> overrode Vim's native number increment and the
visual g<C-a>/g<C-x> variants added complexity for marginal value.
toggle_complete() left the cursor on the wrong line after re-render.

Solution: remove change_priority/change_priority_visual; add
toggle_priority() (0<->1) mapped to '!', with cursor-follow after
render matching the pattern already used in priority toggle. Add
cursor-follow to toggle_complete() for the same reason. Update plugin
plugs (priority-up/down -> priority) and add 'due'/'undo' to the
:Pending completion list. Update help text accordingly.

* feat(buffer): reflect current view in buffer name

Problem: no way to tell at a glance which view (category vs priority)
is active — the buffer was always named 'pending://'.

Solution: update the buffer name to 'pending://category' or
'pending://priority' on every render, so the view is visible in
the statusline/tabline without any extra UI.

lua/pending/init.lua

@@ -52,17 +52,8 @@ function M._setup_buf_mappings(bufnr)
   vim.keymap.set('n', 'g?', function()
     M.show_help()
   end, opts)
-  vim.keymap.set('n', '<C-a>', function()
-    M.change_priority(1)
-  end, opts)
-  vim.keymap.set('n', '<C-x>', function()
-    M.change_priority(-1)
-  end, opts)
-  vim.keymap.set('v', 'g<C-a>', function()
-    M.change_priority_visual(1)
-  end, opts)
-  vim.keymap.set('v', 'g<C-x>', function()
-    M.change_priority_visual(-1)
+  vim.keymap.set('n', '!', function()
+    M.toggle_priority()
   end, opts)
   vim.keymap.set('n', 'D', function()
     M.prompt_date()
@@ -126,10 +117,15 @@ function M.toggle_complete()
   end
   store.save()
   buffer.render(bufnr)
+  for lnum, m in ipairs(buffer.meta()) do
+    if m.id == id then
+      vim.api.nvim_win_set_cursor(0, { lnum, 0 })
+      break
+    end
+  end
 end
 
----@param delta integer
-function M.change_priority(delta)
+function M.toggle_priority()
   local bufnr = buffer.bufnr()
   if not bufnr then
     return
@@ -147,7 +143,7 @@ function M.change_priority(delta)
   if not task then
     return
   end
-  local new_priority = math.max(0, task.priority + delta)
+  local new_priority = task.priority > 0 and 0 or 1
   store.update(id, { priority = new_priority })
   store.save()
   buffer.render(bufnr)
@@ -159,33 +155,6 @@ function M.change_priority(delta)
   end
 end
 
----@param delta integer
-function M.change_priority_visual(delta)
-  local bufnr = buffer.bufnr()
-  if not bufnr then
-    return
-  end
-  local start_row = vim.fn.line("'<")
-  local end_row = vim.fn.line("'>")
-  local meta = buffer.meta()
-  local changed = false
-  for row = start_row, end_row do
-    local m = meta[row]
-    if m and m.type == 'task' and m.id then
-      local task = store.get(m.id)
-      if task then
-        local new_priority = math.max(0, task.priority + delta)
-        store.update(m.id, { priority = new_priority })
-        changed = true
-      end
-    end
-  end
-  if changed then
-    store.save()
-    buffer.render(bufnr)
-  end
-end
-
 function M.prompt_date()
   local bufnr = buffer.bufnr()
   if not bufnr then
@@ -342,10 +311,7 @@ function M.show_help()
     '',
     '<CR>     Toggle complete/uncomplete',
     '<Tab>    Switch category/priority view',
-    '<C-a>    Raise priority level',
-    '<C-x>    Lower priority level',
-    'g<C-a>   Raise priority for visual selection',
-    'g<C-x>   Lower priority for visual selection',
+    '!        Toggle urgent',
     'D        Set due date',
     'U        Undo last write',
     'o / O    Add new task line',
@@ -371,7 +337,7 @@ function M.show_help()
     '',
     'Highlights:',
     '  PendingOverdue   overdue tasks (red)',
-    '  PendingPriority  [N] priority prefix',
+    '  PendingPriority  [!] urgent tasks',
     '',
     'Press q or <Esc> to close',
   }