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/slack/monitor/slash.ts

@@ -136,6 +136,7 @@ export function registerSlackMonitorSlashCommands(params: {
           channelId: command.channel_id,
           channelName: channelInfo?.name,
           channels: ctx.channelsConfig,
+          defaultRequireMention: ctx.defaultRequireMention,
         });
         if (ctx.useAccessGroups) {
           const channelAllowlistConfigured =