feat(todo): comments on a task (ADR 0033, D36)

Plain text with links, written, edited and deleted in the task's sheet,
counted on its row. They are sealed in `task_comment`, the task they belong
to with them, and go with their task.

Whats-New: Comments: write on a task, edit or delete what you wrote, and open its links.
Whats-New-De: Kommentare: Schreibe zu einer Aufgabe, bearbeite oder lösche es und öffne Links darin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0033-halkyon-todo.md

@@ -727,6 +727,8 @@ The developer's review of 4 Nov 2026, again beside Todoist's screens.
 
 The table's rows aren't dragged and its columns aren't chosen: the list is where tasks are placed by hand, and the Display's grouping, sorting and filters apply to it as to the list. Below 720 pixels of its own width, a phone or beside the task pane, a project kept as a table shows as a list, so the same project works on both.
 
+Comments are kept in their own sealed table, `task_comment`, and which task one belongs to is sealed with it, so the instance can't tell how much is said about what. A task deleted takes its comments along on the device that deletes it. Links are http and https alone, opened in the browser: a comment can't start anything else on a device, whatever it says. Links show in comments, not yet in a task's description, which is an editing field.
+
 ## Steps
 
 1. **The frame**: go_router and `NavigationFrame` with the four tabs (empty screens that