Get Sync Chat Context

Per-user corpus-digest context for a sync-chat turn (agent-service-facing).

Authed by the sync-chat-turn JWT. The JWT's user_id scopes the entire response; request inputs cannot widen it. Returns:

• corpus digest: the user's scott-push sessions from the last 14 days with stats and key messages, recency-ordered (newest first, limit 30)
• GitHub install state: so the assistant knows whether to nudge
• thread ancestry: root to from_turn_id (same as review-chat context)