The client posted a body-only envelope to {DISPATCH_API_URL}/messages — an
endpoint that does not exist. Now: /dispatch/envelopes, mandatory DispatchCard
block per the platform pydantic model (int schema_version=1/state_code=0,
task_id=0, int4-safe epoch-second sequence_number), payload under card.event,
UUID card_uid (stable uuid5 for ULID-style asset ids). Inbox accepts both
card.event and legacy root body.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>