feat(slack): add dm-specific replyToMode configuration (#1442)

Adds support for separate replyToMode settings for DMs vs channels:

- Add channels.slack.dm.replyToMode for DM-specific threading
- Keep channels.slack.replyToMode as default for channels
- Add resolveSlackReplyToMode helper to centralize logic
- Pass chatType through threading resolution chain

Usage:
```json5
{
  channels: {
    slack: {
      replyToMode: "off",     // channels
      dm: {
        replyToMode: "all"    // DMs always thread
      }
    }
  }
}
```

When dm.replyToMode is set, DMs use that mode; channels use the
top-level replyToMode. Backward compatible when not configured.

src/auto-reply/reply/reply-threading.test.ts

@@ -31,6 +31,33 @@ describe("resolveReplyToMode", () => {
     expect(resolveReplyToMode(cfg, "discord")).toBe("first");
     expect(resolveReplyToMode(cfg, "slack")).toBe("all");
   });
+
+  it("uses dm-specific replyToMode for Slack DMs when configured", () => {
+    const cfg = {
+      channels: {
+        slack: {
+          replyToMode: "off",
+          dm: { replyToMode: "all" },
+        },
+      },
+    } as ClawdbotConfig;
+    expect(resolveReplyToMode(cfg, "slack", null, "direct")).toBe("all");
+    expect(resolveReplyToMode(cfg, "slack", null, "channel")).toBe("off");
+    expect(resolveReplyToMode(cfg, "slack", null, undefined)).toBe("off");
+  });
+
+  it("falls back to top-level replyToMode when dm.replyToMode is not configured", () => {
+    const cfg = {
+      channels: {
+        slack: {
+          replyToMode: "first",
+          dm: { enabled: true },
+        },
+      },
+    } as ClawdbotConfig;
+    expect(resolveReplyToMode(cfg, "slack", null, "direct")).toBe("first");
+    expect(resolveReplyToMode(cfg, "slack", null, "channel")).toBe("first");
+  });
 });
 
 describe("createReplyToModeFilter", () => {