[Feat] Keep private Sessions out of shared memory

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Telegram.

What changed

• Exclude private tasks and conversations from automatic Brain enqueue, explicit saves, retries, resets, backfill, PR-driven requeue, and both drainers.
• Keep read-only Brain access while removing synthesize and private query/result persistence; deny external shared-memory MCPs.

Why this change was made

Every producer and drainer must reject private content so retries and explicit tools cannot copy it into shared memory.

Impact

Shared Brain behavior remains unchanged; private state is still dark in this stack step.

Related PRs

• #2635 — Private Session access foundation
• #2638 — Privacy inheritance
• #2640 — Private integration binding
• #2642 — Shared egress blocking
• #2643 — Lifecycle and creation UI

[roomote-community]

1 issue outstanding. See task

• Propagate parent Session privacy into attached Fast conversations so private Sessions cannot reach shared-memory paths.

_{Reviewed 060a229}

[roomote-community]

This is the Fast conversation's privacy, not the parent sessions row. A Fast conversation attached to a private Session is inserted with its default shared privacy, and attachFastConversationToSession only sets the foreign key. That leaves the integration filtering/audit redaction disabled and lets save_memory enqueue content for a private Session. Propagate the parent Session privacy when binding, or resolve the bound Session's privacy here.