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/message-handler/prepare.ts

@@ -56,6 +56,7 @@ export async function prepareSlackMessage(params: {
         channelId: message.channel,
         channelName,
         channels: ctx.channelsConfig,
+        defaultRequireMention: ctx.defaultRequireMention,
       })
     : null;
 
@@ -200,7 +201,9 @@ export async function prepareSlackMessage(params: {
     cfg,
     surface: "slack",
   });
-  const shouldRequireMention = isRoom ? (channelConfig?.requireMention ?? true) : false;
+  const shouldRequireMention = isRoom
+    ? (channelConfig?.requireMention ?? ctx.defaultRequireMention)
+    : false;
 
   // Allow "control commands" to bypass mention gating if sender is authorized.
   const shouldBypassMention =