feat(v2): support async channel adapter factories

Channel adapter factories can now return a Promise, enabling adapters
that need async initialization like loading auth state from disk
(e.g. WhatsApp reading credentials via useMultiFileAuthState).
Existing sync factories are unaffected — await on a sync return is
a no-op. All current adapters remain synchronous.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/channels/adapter.ts

@@ -97,7 +97,7 @@ export interface ChannelAdapter {
 }
 
 /** Factory function that creates a channel adapter (returns null if credentials missing). */
-export type ChannelAdapterFactory = () => ChannelAdapter | null;
+export type ChannelAdapterFactory = () => ChannelAdapter | Promise<ChannelAdapter> | null;
 
 /** Registration entry for a channel adapter. */
 export interface ChannelRegistration {