A Matrix `attach_to_session` cron delivery and a CLI→Matrix `/handoff` now land in their own
thread AND a human reply in that thread continues the seeded session (#112918).
The salvaged `MatrixAdapter.create_handoff_thread` (ea-s21, #108367) gives both seeders a
thread root. The second half of the bug is a key-shape mismatch: the handoff watcher and
`_seed_cron_thread_session` seeded `matrix🧵<room>:<root>` while the adapter keys every
in-thread reply on the ROOM's type (`matrix:group|dm:<room>:<root>`), so seed and reply never
met. Following the direction landed for Slack in 1f3f45e87b (#111896), the seeders now mirror
the adapter instead of the adapter moving onto a `thread` slot: rekeying inbound Matrix threads
would orphan every existing Matrix thread session and drop `is_group` in authz for in-thread
messages (`_GROUP_CHAT_TYPES` has no `thread`).
- gateway/run_startup.py: Matrix destinations key `dm`/`group` from the adapter's
`get_chat_info` (`_handoff_home_is_dm`).
- cron/scheduler_delivery.py: `_THREAD_REPLY_CHAT_TYPE` — Slack and Matrix non-DM thread seeds
use `group`. Slack channel cron threads had the same mismatch (adapter `build_source` keys
`group`; the seed said `thread`); the two cron tests that pinned `thread` for Slack channels
asserted the wrong shape and now assert the adapter's.
- Dropped from #108367: the `chat_type="thread"` inbound rekey (see above) and its tests;
contributor tests trimmed to two invariants.
- Docs: Matrix listed among thread-capable handoff/cron platforms.
Live probe (in-process, real MatrixAdapter + real handoff destination + real cron seeder):
before create_handoff_thread -> None; handoff key matrix🧵… ≠ inbound matrix:group:…;
cron seed matrix🧵… ≠ inbound
after create_handoff_thread -> '$seed'; handoff == inbound == cron seed (room and DM cases)
Co-authored-by: ea-s21 <190767603+ea-s21@users.noreply.github.com>
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