Sync Chat Write Up Workspace

Trigger a tailored write-up on one of the caller's own push planspaces.

Both endpoints are strictly user-scoped: a sync-chat turn can pin or write up ONLY the caller's own push planspaces. A workspace_id that is not one of the caller's own scott_push_artifacts (matched on captured_by_user_id == user_id) returns 403.

Appends a single pending synthesis turn off the planspace's scott-push chain tip (the planspace was bulk-uploaded with synthesize=false), then dispatches it to agent-service after commit, the same way push-ingest synthesis fans out.