Add channel_keys dict to NetworkConfig for storing per-channel keys. Keys are used in KICK rejoin, passed via AUTOJOIN +#channel key syntax, supported in ADDNETWORK channel_keys= parameter, and propagated through REHASH. Extract rehash() as reusable async function for SIGHUP reuse. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>