Add Zulip standard [api] section config support and daily changelog
ocaml-zulip: Support Zulip's native config format which uses [api] section
with "key" field instead of [bot] section with "api_key". Config loading
now tries [bot], then [api], then bare format.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>