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.tool-result.sends-tool-summaries-responseprefix.test.ts

@@ -423,6 +423,49 @@ describe("monitorSlackProvider tool results", () => {
     expect(replyMock.mock.calls[0][0].WasMentioned).toBe(true);
   });
 
+  it("accepts channel messages without mention when channels.slack.requireMention is false", async () => {
+    config = {
+      channels: {
+        slack: {
+          dm: { enabled: true, policy: "open", allowFrom: ["*"] },
+          groupPolicy: "open",
+          requireMention: false,
+        },
+      },
+    };
+    replyMock.mockResolvedValue({ text: "hi" });
+
+    const controller = new AbortController();
+    const run = monitorSlackProvider({
+      botToken: "bot-token",
+      appToken: "app-token",
+      abortSignal: controller.signal,
+    });
+
+    await waitForEvent("message");
+    const handler = getSlackHandlers()?.get("message");
+    if (!handler) throw new Error("Slack message handler not registered");
+
+    await handler({
+      event: {
+        type: "message",
+        user: "U1",
+        text: "hello",
+        ts: "123",
+        channel: "C1",
+        channel_type: "channel",
+      },
+    });
+
+    await flush();
+    controller.abort();
+    await run;
+
+    expect(replyMock).toHaveBeenCalledTimes(1);
+    expect(replyMock.mock.calls[0][0].WasMentioned).toBe(false);
+    expect(sendMock).toHaveBeenCalledTimes(1);
+  });
+
   it("treats control commands as mentions for group bypass", async () => {
     replyMock.mockResolvedValue({ text: "ok" });