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/channel-config.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+
+import { resolveSlackChannelConfig } from "./channel-config.js";
+
+describe("resolveSlackChannelConfig", () => {
+  it("uses defaultRequireMention when channels config is empty", () => {
+    const res = resolveSlackChannelConfig({
+      channelId: "C1",
+      channels: {},
+      defaultRequireMention: false,
+    });
+    expect(res).toEqual({ allowed: true, requireMention: false });
+  });
+
+  it("defaults defaultRequireMention to true when not provided", () => {
+    const res = resolveSlackChannelConfig({
+      channelId: "C1",
+      channels: {},
+    });
+    expect(res).toEqual({ allowed: true, requireMention: true });
+  });
+
+  it("prefers explicit channel/fallback requireMention over defaultRequireMention", () => {
+    const res = resolveSlackChannelConfig({
+      channelId: "C1",
+      channels: { "*": { requireMention: true } },
+      defaultRequireMention: false,
+    });
+    expect(res).toMatchObject({ requireMention: true });
+  });
+});
+