fix(slack): respect top-level requireMention config

The `channels.slack.requireMention` setting was defined in the schema
but never passed to `resolveSlackChannelConfig()`, which always
defaulted to `true`. This meant setting `requireMention: false` at the
top level had no effect—channels still required mentions.

Pass `slackCfg.requireMention` as `defaultRequireMention` to the
resolver and use it as the fallback instead of hardcoded `true`.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/config/zod-schema.providers-core.ts

@@ -212,6 +212,7 @@ export const SlackAccountSchema = z.object({
   botToken: z.string().optional(),
   appToken: z.string().optional(),
   allowBots: z.boolean().optional(),
+  requireMention: z.boolean().optional(),
   groupPolicy: GroupPolicySchema.optional().default("allowlist"),
   historyLimit: z.number().int().min(0).optional(),
   dmHistoryLimit: z.number().int().min(0).optional(),