_THREAD_REPLY_CHAT_TYPE covered Slack and Matrix but left Telegram on the default "thread"
slot, while the Telegram adapter's _build_message_event types every supergroup message
"group" (forum topics included), so a forum-supergroup continuable cron seed landed in
agent:main:telegram🧵<chat>:<topic> and the user's topic reply resolved to
agent:main:telegram:group:<chat>:<topic> — the same seed/reply split the Matrix fix closes,
on the untouched sibling platform (and the slot the /handoff path already binds).
Add telegram to the table; retarget the two tests that pinned the wrong "thread" contract
for a Telegram seed (the backward-compat default now uses Discord, the one platform whose
in-thread replies key "thread"); add one seed-key == reply-key invariant test for the
Telegram forum topic. Mirror the three one-line Matrix doc edits into the zh-Hans pages.