feat: collaboration summaries

[ryanweiler92]

Collaboration: background thread summaries and action items

Work threads' summaries and action items are now made on the server, in the background, by Brain's text model (COLLAB_LLM_ENGINE_ID). Summarize no longer sends a turn through the thread's assistant room, so the room's conversation and running task are untouched.

When it runs

• New mail: BrainSync queues every thread with new mail (except automated or skipped threads) after classifying. The sync does not wait for the summaries.
• On open: WorkSummarizeThread(threadId) runs right away if the summary does not cover the thread's newest message. Otherwise it returns at once.
• On Summarize / Regenerate: WorkSummarizeThread(threadId, force=true) always writes a new one.
• WorkGetThreadInsights(threadId) returns the summary, whether it is current, run status (running / done / failed + error), and the thread's steps.

Runs use two small daemon pools of their own: one for synced mail and one for threads the owner has open. A thread has at most one run at a time.

Keeping action items current without duplicates

• The model sees the steps already tracked and returns the current list. Generated steps are updated in place, or closed when a reply shows they're done. Untouched generated steps it leaves out are removed.
• An item without an id that matches a tracked or dismissed step (same or nearly the same words) is treated as that step, not added again.
• The owner's own steps, edited steps, finished steps and dismissed steps are never rewritten or re-added. An edited generated step can still be closed.
• Re-reading mail the summary already covers does nothing.
• Deleting a generated step keeps it as dismissed (unlisted), so it isn't suggested again.

Schema (added automatically on startup)

• BRAIN_THREAD.SUMMARY_REF, SUMMARY_AT: the newest message the summary covers, and when it was written.
• WORK_THREAD_STEP.ORIGIN (brain) and EDITED (set when the owner changes text, owner, due or kind).

Other changes

• BrainListThreads(detail=true) adds summaryAt, summaryCurrent and summaryPending. Steps include origin.
• Including or excluding a participant, or adding or removing a rule, marks summaries stale, so they rerun on the next open or new mail.
• The BrainTopicModel access error now says "Brain's text model" instead of "topic model".

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