v0.21.9

Fixed

• fix(channel): suppress empty-content delivery acks at the sender
  (#260, lead handoff). Killed the empty-ack ping-pong spam hitting
  every inbox (incl. lead's). New _channel_ack_filter +
  _channel_auto_ack modules; sender-side filter drops empty-
  content acks before the wire so the receiver never sees them.
  Rate-limit env knobs (SAC_AUTO_ACK_RATE_MAX /
  SAC_AUTO_ACK_RATE_WINDOW_S) gate spammy auto-ack loops.
• fix(smoke): update deny-row assertions for task #27 two-row
  contract (#281). v0.21.8's pypa-publish failed because the
  smoke test test_listen_denied_send_persists_exactly_one_ channel_events_row pinned the pre-task-#27 single-row
  contract; task #27 (block/unblock approve-flow, landed in
  v0.21.8's contents) intentionally added a second row (the
  approval_prompt push). Smoke test updated to the new two-
  row contract + 5 new assertions on the prompt body (embeds
  both sac a2a unblock and sac a2a block, does NOT leak
  the sender's body). No production code change; v0.21.8's
  feature set still ships here.

v0.21.9 = v0.21.8's full feature set (#278 + #279, task #27 ACL
approve-prompt flow) re-cut after #260 + #281 unblocked the
release pipeline. The v0.21.8 tag exists on GitHub but is a
"ghost" — no PyPI artifact and no GitHub Release; operators
install scitex-agent-container==0.21.9 for the same feature
set + the empty-ack fix + the smoke-test contract update.