You cannot tag a task or add subtasks, and a mentioned colleague is never notified #649

What happens

On a task's detail page you cannot attach a label or add a subtask. The board can filter by label, but no screen lets you put one on a task. When you @mention a colleague in a comment, the mention renders, but the colleague gets no notification.

Why

Read at development 6fa5ef5.

• src/utils/taskHelpers.js:34 sets TASK_SIDEBAR_HIDDEN_TABS = ['tags', 'tasks'], so the task sidebar hides the library's tags and tasks tabs. The comment says Planninq has its own label and subtask concepts, but no replacement exists. task.labels is only displayed and filtered. task.parent is never read or written in src/.
• src/views/TaskDetail.vue:129-137 mounts CnObjectSidebar without a @mention listener. In nextcloud-vue, CnNotesTab emits mention with the mentioned user ids and dispatches nothing itself (CnNotesTab.vue:164-173). CnObjectSidebar passes it through. The consuming app has to notify, and Planninq does not.

Fix direction

1. Either stop hiding the tags tab, or add a label picker to the task detail page that writes task.labels.
2. Either stop hiding the tasks tab, or add a subtask list that writes task.parent.
3. Bind @mention on the sidebar and send a Nextcloud notification from the backend to each mentioned user who is a project member.

Live check

1. Open a task. Look for a way to add a label or a subtask. Expected today: none.
2. As user A, comment "@b please look" on a task in a project B belongs to. Log in as B. Expected today: no notification.

Matrix rows

tsk-labels, tsk-subtasks, tsk-mentions in openspec/parity/capabilities.json (PR #645).