feat(views): add hide_done_categories config option (#153)

Problem: Categories where every task is done still render in the buffer,
cluttering the view when entire categories are finished.

Solution: Add `view.category.hide_done_categories` (boolean, default
false). When enabled, `category_view()` skips categories whose tasks are
all done/deleted, returns their IDs as `done_cat_hidden_ids`, and
`_on_write` merges those IDs into `hidden_ids` passed to `diff.apply()`
so hidden tasks are not mistakenly deleted on `:w`.

spec/views_spec.lua

@@ -280,6 +280,85 @@ describe('views', function()
       assert.are.equal('# Alpha', headers[1])
       assert.are.equal('# Beta', headers[2])
     end)
+
+    it('returns empty done_cat_hidden_ids when hide_done_categories is false', function()
+      local t1 = s:add({ description = 'Done task', category = 'Work' })
+      s:update(t1.id, { status = 'done' })
+      s:add({ description = 'Active', category = 'Personal' })
+      local _, _, done_hidden = views.category_view(s:active_tasks())
+      assert.are.same({}, done_hidden)
+    end)
+
+    it('hides categories with only done tasks when hide_done_categories is true', function()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        view = { category = { hide_done_categories = true } },
+      }
+      config.reset()
+      local t1 = s:add({ description = 'Done task', category = 'Work' })
+      s:update(t1.id, { status = 'done' })
+      s:add({ description = 'Active', category = 'Personal' })
+      local lines, meta, done_hidden = views.category_view(s:active_tasks())
+      local headers = {}
+      for i, m in ipairs(meta) do
+        if m.type == 'header' then
+          table.insert(headers, lines[i])
+        end
+      end
+      assert.are.equal(1, #headers)
+      assert.are.equal('# Personal', headers[1])
+      assert.are.same({ [t1.id] = true }, done_hidden)
+    end)
+
+    it('shows categories with a mix of done and pending tasks', function()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        view = { category = { hide_done_categories = true } },
+      }
+      config.reset()
+      local t1 = s:add({ description = 'Done task', category = 'Work' })
+      s:update(t1.id, { status = 'done' })
+      s:add({ description = 'Active task', category = 'Work' })
+      local lines, meta, done_hidden = views.category_view(s:active_tasks())
+      local headers = {}
+      for i, m in ipairs(meta) do
+        if m.type == 'header' then
+          table.insert(headers, lines[i])
+        end
+      end
+      assert.are.equal(1, #headers)
+      assert.are.equal('# Work', headers[1])
+      assert.are.same({}, done_hidden)
+    end)
+
+    it('does not insert leading blank line when first category is hidden', function()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        view = { category = { hide_done_categories = true } },
+      }
+      config.reset()
+      local t1 = s:add({ description = 'Done task', category = 'Alpha' })
+      s:update(t1.id, { status = 'done' })
+      s:add({ description = 'Active', category = 'Beta' })
+      local lines, meta = views.category_view(s:active_tasks())
+      assert.are.equal('header', meta[1].type)
+      assert.are.equal('# Beta', lines[1])
+    end)
+
+    it('returns all done task ids from hidden categories', function()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        view = { category = { hide_done_categories = true } },
+      }
+      config.reset()
+      local t1 = s:add({ description = 'Done A', category = 'Work' })
+      local t2 = s:add({ description = 'Done B', category = 'Work' })
+      s:update(t1.id, { status = 'done' })
+      s:update(t2.id, { status = 'done' })
+      s:add({ description = 'Active', category = 'Personal' })
+      local _, _, done_hidden = views.category_view(s:active_tasks())
+      assert.are.same({ [t1.id] = true, [t2.id] = true }, done_hidden)
+    end)
   end)
 
   describe('priority_view', function()