feat(buffer): persist fold state across sessions

Problem: folded category headers are lost when Neovim exits because
`_fold_state` only lives in memory. Users must re-fold categories
every session.

Solution: store folded category names in the JSON data file as a
top-level `folded_categories` field. On first render, `restore_folds`
seeds from the store instead of the empty in-memory state. Folds are
persisted on `M.close()` and `VimLeavePre`.

spec/store_spec.lua

@@ -214,6 +214,35 @@ describe('store', function()
     end)
   end)
 
+  describe('folded_categories', function()
+    it('defaults to empty table when missing from JSON', function()
+      local path = tmpdir .. '/tasks.json'
+      local f = io.open(path, 'w')
+      f:write(vim.json.encode({
+        version = 1,
+        next_id = 1,
+        tasks = {},
+      }))
+      f:close()
+      s:load()
+      assert.are.same({}, s:get_folded_categories())
+    end)
+
+    it('round-trips folded categories through save and load', function()
+      s:set_folded_categories({ 'Work', 'Home' })
+      s:save()
+      s:load()
+      assert.are.same({ 'Work', 'Home' }, s:get_folded_categories())
+    end)
+
+    it('persists empty list', function()
+      s:set_folded_categories({})
+      s:save()
+      s:load()
+      assert.are.same({}, s:get_folded_categories())
+    end)
+  end)
+
   describe('active_tasks', function()
     it('excludes deleted tasks', function()
       s:add({ description = 'Active' })