- Guard the thread-id-as-chat_id normalization to Discord only; Slack
and Telegram adapters use parent_channel as chat_id for thread messages,
so the unconditional version broke their handoff keys.
- Apply the same Discord-specific guard to _seed_cron_thread_session in
cron/scheduler.py (sibling site with the same bug, docstring said
'Mirrors _process_handoff').
- Replace the change-detector test with contract tests that verify the
actual invariant: Discord handoff key == organic thread key, Slack
handoff key still uses parent channel (non-regression).
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