Hide sysdedup HTML comments from task body

Seen while visually checking Task 12271 on a local stack: a system-filed broken-session task (body ends with `occurrences: 1 <!-- sysdedup:runner-break:vanished:7 -->`) renders the HTML comment as literal text in the task page's body editor. - Hide machine dedup markers (`<!-- sysdedup:… -->`) from the rendered/edited task body, while keeping them in the stored body so dedup still works. - Check other system-filed bodies that carry HTML comments render cleanly the same way. Out of scope: changing how system tasks are deduplicated. Done means: - A task whose body carries a `<!-- sysdedup:… -->` marker shows no comment text on the task page; the marker is still in the stored body and dedup still matches it (test).

Hid sysdedup and triage markers from the task body editor and renderer; they stay in the stored body and every edit re-appends them. Quick gate and richtext/tasks tests are green; the visual check at 1280px and 448px is clean. PR #7630 is queued in the merge lane. Memory: none applied.