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.

spec/views_spec.lua

@@ -27,7 +27,7 @@ describe('views', function()
       store.add({ description = 'Task A', category = 'Work' })
       store.add({ description = 'Task B', category = 'Work' })
       local lines, meta = views.category_view(store.active_tasks())
-      assert.are.equal('Work', lines[1])
+      assert.are.equal('## Work', lines[1])
       assert.are.equal('header', meta[1].type)
       assert.is_true(lines[2]:find('Task A') ~= nil)
       assert.is_true(lines[3]:find('Task B') ~= nil)
@@ -113,10 +113,10 @@ describe('views', function()
           task_line = lines[i]
         end
       end
-      assert.are.equal('/1/  My task', task_line)
+      assert.are.equal('/1/- [ ] My task', task_line)
     end)
 
-    it('formats priority task lines as /ID/  [N] description', function()
+    it('formats priority task lines as /ID/- [!] description', function()
       store.add({ description = 'Important', category = 'Inbox', priority = 1 })
       local lines, meta = views.category_view(store.active_tasks())
       local task_line
@@ -125,7 +125,7 @@ describe('views', function()
           task_line = lines[i]
         end
       end
-      assert.are.equal('/1/  [1] Important', task_line)
+      assert.are.equal('/1/- [!] Important', task_line)
     end)
 
     it('sets LineMeta type=header for header lines with correct category', function()
@@ -220,8 +220,8 @@ describe('views', function()
           end
         end
       end
-      assert.are.equal('Work', first_header)
-      assert.are.equal('Inbox', second_header)
+      assert.are.equal('## Work', first_header)
+      assert.are.equal('## Inbox', second_header)
     end)
 
     it('appends categories not in category_order after ordered ones', function()
@@ -236,8 +236,8 @@ describe('views', function()
           table.insert(headers, lines[i])
         end
       end
-      assert.are.equal('Work', headers[1])
-      assert.are.equal('Errands', headers[2])
+      assert.are.equal('## Work', headers[1])
+      assert.are.equal('## Errands', headers[2])
     end)
 
     it('preserves insertion order when category_order is empty', function()
@@ -250,8 +250,8 @@ describe('views', function()
           table.insert(headers, lines[i])
         end
       end
-      assert.are.equal('Alpha', headers[1])
-      assert.are.equal('Beta', headers[2])
+      assert.are.equal('## Alpha', headers[1])
+      assert.are.equal('## Beta', headers[2])
     end)
   end)
 
@@ -325,10 +325,10 @@ describe('views', function()
       assert.is_true(earlier_row < later_row)
     end)
 
-    it('formats task lines as /ID/  description', function()
+    it('formats task lines as /ID/- [ ] description', function()
       store.add({ description = 'My task', category = 'Inbox' })
       local lines, _ = views.priority_view(store.active_tasks())
-      assert.are.equal('/1/  My task', lines[1])
+      assert.are.equal('/1/- [ ] My task', lines[1])
     end)
 
     it('sets show_category=true for all task meta entries', function()