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>

apps/todo/halkyon_todo_client/lib/migrations/migration_registry.dart

@@ -8,6 +8,7 @@ part '20261101000019144-projects/migration.dart';
 part '20261101000604776-tasks/migration.dart';
 part '20261101180755366-settings/migration.dart';
 part '20261106183802735-filters/migration.dart';
+part '20261106184947570-comments/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -18,6 +19,7 @@ class MigrationRegistry {
     _Migration20261101000604776_tasks(),
     _Migration20261101180755366_settings(),
     _Migration20261106183802735_filters(),
+    _Migration20261106184947570_comments(),
   ];
 
   /// List of all client-side database migration versions.