When a user deletes a comment that triggered an agent task, the agent
would still run with the now-deleted content baked into its prompt
(fetched at task claim time) — manifesting as "the agent still sees the
deleted comment". The FK ON DELETE SET NULL only nullified
trigger_comment_id; the queued task itself was never cancelled.
DeleteComment now cancels any queued/dispatched/running task whose
trigger is the deleted comment, before the comment row is removed.