Pre-gateway PMs/Board sent formal notifications (require ack); gateway
had say/dm only. Now PMs and Board can issue ack-required notifications
via NotificationService through the standard envelope path.
I1: post_to_channel raised no error if get_agent_slug returned None
(unknown/deleted agent), and send_message's 'if agent_slug' skipped
the validate_channel_access call. Same fail-open class the prior
commit was fixing, just narrower window. Now post_to_channel raises
ChannelAccessDeniedError directly when the slug lookup fails — say()
already converts that to a clean not_authorized Envelope.
I2: 'writable channels for your role' wording was misleading because
get_agent_channels resolves by slug, not role. Replaced with
'channels you may write to' for clarity.
post_to_channel was calling send_message without agent_slug, which
disabled the validate_channel_access check. Forward the slug; convert
ChannelAccessDeniedError to a friendly not_authorized Envelope with
the agent's writable-channel list (pre-gateway behaviour).
Renn F