Promote a fast session

Promotes a lightweight fast (quick-ask) conversation into a full standard conversation, binding it to the workspace’s default agent so subsequent turns run in the full agent environment seeded with the existing transcript. The caller must be a participant of the conversation (returns 403 otherwise). The promotion is one-way and idempotent: it is a no-op if the conversation is already standard. Requires the conversations:write scope. Promote a fast session to a full hq: flip mode off fast, one-way. The conversation already holds its transcript, so the next turn in Studio cold-boots a VM seeded with that history via the normal path - no backfill. Idempotent: a no-op if it was already standard.