fix(discord): add missing autoThread to DiscordGuildChannelConfig type

The autoThread field is present in the Zod schema
(DiscordGuildChannelSchema) and actively used at runtime in
threading.ts and message-handler.process.ts, but was missing
from the canonical TypeScript type definition. This forced
runtime code in allow-list.ts to define its own local type
with the field.

Add autoThread to DiscordGuildChannelConfig to align the type
with the schema and runtime usage.

src/config/types.discord.ts

@@ -52,6 +52,8 @@ export type DiscordGuildChannelConfig = {
   systemPrompt?: string;
   /** If false, omit thread starter context for this channel (default: true). */
   includeThreadStarter?: boolean;
+  /** If true, automatically create a thread for each new message in this channel. */
+  autoThread?: boolean;
 };
 
 export type DiscordReactionNotificationMode = "off" | "own" | "all" | "allowlist";