Add a generic per-platform PlatformConfig.typing_indicator flag (default
True) that gates the _keep_typing refresh loop in
_process_message_background. When false, the loop is never spawned, so no
typing/"is thinking…" status is shown on that platform — message delivery
is otherwise unchanged.
Mirrors the gateway_restart_notification contract exactly: dataclass field
+ to_dict/from_dict (with extra-fallback resolution) + shared-key bridge in
load_gateway_config, so 'slack: typing_indicator: false' under platforms
works without a separate block. Generic by design — the same key works for
every platform (Slack 'is thinking…', Telegram/Discord/Signal typing).
Motivated by users who find Slack's assistant 'is thinking…' status noisy
(it also briefly disables the compose box, via the Assistant API).
