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/provider.ts

@@ -122,6 +122,7 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
     allowFrom,
     groupDmEnabled,
     groupDmChannels,
+    defaultRequireMention: slackCfg.requireMention,
     channelsConfig,
     groupPolicy,
     useAccessGroups,