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/diff.lua

@@ -7,6 +7,7 @@ local store = require('pending.store')
 ---@field id? integer
 ---@field description? string
 ---@field priority? integer
+---@field status? string
 ---@field category? string
 ---@field due? string
 ---@field lnum integer
@@ -26,20 +27,17 @@ function M.parse_buffer(lines)
   local current_category = nil
 
   for i, line in ipairs(lines) do
-    local id, body = line:match('^/(%d+)/(  .+)$')
+    local id, body = line:match('^/(%d+)/(- %[.%] .*)$')
     if not id then
-      body = line:match('^(  .+)$')
+      body = line:match('^(- %[.%] .*)$')
     end
     if line == '' then
       table.insert(result, { type = 'blank', lnum = i })
     elseif id or body then
-      local stripped = body:match('^  (.+)$') or body
-      local prio_str = stripped:match('^%[(%d+)%] ')
-      local priority = 0
-      if prio_str then
-        priority = tonumber(prio_str) --[[@as integer]]
-        stripped = stripped:sub(#prio_str + 4)
-      end
+      local stripped = body:match('^- %[.%] (.*)$') or body
+      local state_char = body:match('^- %[(.-)%]') or ' '
+      local priority = state_char == '!' and 1 or 0
+      local status = state_char == 'x' and 'done' or 'pending'
       local description, metadata = parse.body(stripped)
       if description and description ~= '' then
         table.insert(result, {
@@ -47,14 +45,15 @@ function M.parse_buffer(lines)
           id = id and tonumber(id) or nil,
           description = description,
           priority = priority,
+          status = status,
           category = metadata.cat or current_category or config.get().default_category,
           due = metadata.due,
           lnum = i,
         })
       end
-    elseif line:match('^%S') then
-      current_category = line
-      table.insert(result, { type = 'header', category = line, lnum = i })
+    elseif line:match('^## (.+)$') then
+      current_category = line:match('^## (.+)$')
+      table.insert(result, { type = 'header', category = current_category, lnum = i })
     end
   end
 
@@ -113,6 +112,15 @@ function M.apply(lines)
           task.due = entry.due
           changed = true
         end
+        if entry.status and task.status ~= entry.status then
+          task.status = entry.status
+          if entry.status == 'done' then
+            task['end'] = now
+          else
+            task['end'] = nil
+          end
+          changed = true
+        end
         if task.order ~= order_counter then
           task.order = order_counter
           changed = true