The channel gate ran inside _handle_slash_command, after the bolt listener had
already acked with an ephemeral "Running /x". In an ignored or non-allowed
channel the invoker was promised a run that never came, and an ignored channel
("never touched") still got a bot reply. Move both checks into
_slash_channel_gated() and consult it before the ack: gated -> bare ack().
_handle_slash_command keeps calling the same predicate for direct callers.
The listener becomes the _handle_hermes_command method so the existing
parametrized gate test drives the real ack path (BN11 gate r1 2c Low finding).
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