_setup_slack() was the only platform setup function that did not prompt
for a home channel. All four sibling setups (_setup_telegram,
_setup_discord, _setup_mattermost, _setup_bluebubbles) close with an
identical home-channel block, and setup_gateway() already checks for
SLACK_HOME_CHANNEL presence at the end of the wizard — but the value
was never collected, leaving cron delivery and cross-platform
notifications silently broken for Slack after a fresh hermes setup run.
Add the standard home-channel prompt at the end of _setup_slack(),
symmetric with the Discord implementation. Add two unit tests that
verify the prompt is saved when provided and skipped when left blank.
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