- A mid-turn notify reply (/status, /approve, clarify answer) shares the
stream's thread key; it no longer seals and overwrites the half-streamed
answer. In-place replacement now requires the final to match the stream
after normalizing mrkdwn markers and whitespace; anything else posts fresh
and leaves the stream open.
- One _commit_stream helper for both seal-then-commit paths, so the rewrite
path also falls back to chat.update when stopStream fails.
- A stream reopened after a server-side seal is seeded with only the text
past the sealed message (tracked as 'base'), not the whole segment.
- Streams older than 15 min are sealed and dropped on the next start.
- _stream_key reuses _workspace_thread_key/scope_id_for_chat; the stream
dict no longer duplicates chat/team ids.
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