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/slack/threading-tool-context.test.ts

@@ -0,0 +1,96 @@
+import { describe, expect, it } from "vitest";
+
+import type { ClawdbotConfig } from "../config/config.js";
+import { buildSlackThreadingToolContext } from "./threading-tool-context.js";
+
+const emptyCfg = {} as ClawdbotConfig;
+
+describe("buildSlackThreadingToolContext", () => {
+  it("uses top-level replyToMode by default", () => {
+    const cfg = {
+      channels: {
+        slack: { replyToMode: "first" },
+      },
+    } as ClawdbotConfig;
+    const result = buildSlackThreadingToolContext({
+      cfg,
+      accountId: null,
+      context: { ChatType: "channel" },
+    });
+    expect(result.replyToMode).toBe("first");
+  });
+
+  it("uses dm.replyToMode for direct messages when configured", () => {
+    const cfg = {
+      channels: {
+        slack: {
+          replyToMode: "off",
+          dm: { replyToMode: "all" },
+        },
+      },
+    } as ClawdbotConfig;
+    const result = buildSlackThreadingToolContext({
+      cfg,
+      accountId: null,
+      context: { ChatType: "direct" },
+    });
+    expect(result.replyToMode).toBe("all");
+  });
+
+  it("uses top-level replyToMode for channels even when dm.replyToMode is set", () => {
+    const cfg = {
+      channels: {
+        slack: {
+          replyToMode: "off",
+          dm: { replyToMode: "all" },
+        },
+      },
+    } as ClawdbotConfig;
+    const result = buildSlackThreadingToolContext({
+      cfg,
+      accountId: null,
+      context: { ChatType: "channel" },
+    });
+    expect(result.replyToMode).toBe("off");
+  });
+
+  it("falls back to top-level when dm.replyToMode is not set", () => {
+    const cfg = {
+      channels: {
+        slack: {
+          replyToMode: "first",
+          dm: { enabled: true },
+        },
+      },
+    } as ClawdbotConfig;
+    const result = buildSlackThreadingToolContext({
+      cfg,
+      accountId: null,
+      context: { ChatType: "direct" },
+    });
+    expect(result.replyToMode).toBe("first");
+  });
+
+  it("uses all mode when ThreadLabel is present", () => {
+    const cfg = {
+      channels: {
+        slack: { replyToMode: "off" },
+      },
+    } as ClawdbotConfig;
+    const result = buildSlackThreadingToolContext({
+      cfg,
+      accountId: null,
+      context: { ChatType: "channel", ThreadLabel: "some-thread" },
+    });
+    expect(result.replyToMode).toBe("all");
+  });
+
+  it("defaults to off when no replyToMode is configured", () => {
+    const result = buildSlackThreadingToolContext({
+      cfg: emptyCfg,
+      accountId: null,
+      context: { ChatType: "direct" },
+    });
+    expect(result.replyToMode).toBe("off");
+  });
+});