Picks up Subscriptions field on create request types
(CreateMessageRequest, CreateDocumentRequest, CreateScheduleEntryRequest).

Controls who gets subscribed and notified when creating messages, docs,
and schedule entries. Critical for bot/agent use cases that need silent
creation.

  --no-subscribe        subscribe nobody (no notifications)
  --subscribe "X,Y"     subscribe specific people (names, emails, IDs, "me")

Mutually exclusive. Neither flag preserves server default (everyone).
Explicit --subscribe with no resolvable people is a hard error, including
--subscribe "" (via cmd.Flags().Changed detection).

Available on: message, messages create, docs create, schedule create.

Closes #182