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/buffer.lua

@@ -58,9 +58,9 @@ local function setup_syntax(bufnr)
     vim.cmd([[
       syntax clear
       syntax match taskId /^\/\d\+\// conceal
-      syntax match taskHeader /^\S.*$/ contains=taskId
-      syntax match taskPriority /\[\d\+\] / contained containedin=taskLine
-      syntax match taskLine /^\/\d\+\/  .*$/ contains=taskId,taskPriority
+      syntax match taskHeader /^## .*$/ contains=taskId
+      syntax match taskCheckbox /\[!\]/ contained containedin=taskLine
+      syntax match taskLine /^\/\d\+\/- \[.\] .*$/ contains=taskId,taskCheckbox
     ]])
   end)
 end
@@ -74,8 +74,8 @@ function M.open_line(above)
   local row = vim.api.nvim_win_get_cursor(0)[1]
   local insert_row = above and (row - 1) or row
   vim.bo[bufnr].modifiable = true
-  vim.api.nvim_buf_set_lines(bufnr, insert_row, insert_row, false, { '  ' })
-  vim.api.nvim_win_set_cursor(0, { insert_row + 1, 2 })
+  vim.api.nvim_buf_set_lines(bufnr, insert_row, insert_row, false, { '- [ ] ' })
+  vim.api.nvim_win_set_cursor(0, { insert_row + 1, 6 })
   vim.cmd('startinsert!')
 end
 
@@ -115,18 +115,18 @@ local function apply_extmarks(bufnr, line_meta)
         if virt_text then
           vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
             virt_text = virt_text,
-            virt_text_pos = 'right_align',
+            virt_text_pos = 'eol',
           })
         end
       elseif m.due then
         vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
           virt_text = { { m.due, due_hl } },
-          virt_text_pos = 'right_align',
+          virt_text_pos = 'eol',
         })
       end
       if m.status == 'done' then
         local line = vim.api.nvim_buf_get_lines(bufnr, row, row + 1, false)[1] or ''
-        local col_start = line:find('/%d+/') and select(2, line:find('/%d+/')) + 2 or 0
+        local col_start = line:find('/%d+/') and select(2, line:find('/%d+/')) or 0
         vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, col_start, {
           end_col = #line,
           hl_group = 'PendingDone',
@@ -200,6 +200,7 @@ function M.render(bufnr)
   end
 
   current_view = current_view or config.get().default_view
+  vim.api.nvim_buf_set_name(bufnr, 'pending://' .. current_view)
   local tasks = store.active_tasks()
 
   local lines, line_meta
@@ -213,8 +214,11 @@ function M.render(bufnr)
 
   snapshot_folds(bufnr)
   vim.bo[bufnr].modifiable = true
+  local saved = vim.bo[bufnr].undolevels
+  vim.bo[bufnr].undolevels = -1
   vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
   vim.bo[bufnr].modified = false
+  vim.bo[bufnr].undolevels = saved
 
   setup_syntax(bufnr)
   apply_extmarks(bufnr, line_meta)
@@ -261,7 +265,6 @@ function M.open()
   end
 
   task_bufnr = vim.api.nvim_create_buf(true, false)
-  vim.api.nvim_buf_set_name(task_bufnr, 'pending://')
 
   set_buf_options(task_bufnr)
   vim.api.nvim_set_current_buf(task_bufnr)