Slack: implement replyToMode threading for tool path

- Add shared hasRepliedRef state between auto-reply and tool paths
- Extract buildSlackThreadingContext helper in agent-runner.ts
- Extract resolveThreadTsFromContext helper in slack-actions.ts
- Update docs with clear replyToMode table (off/first/all)
- Add tests for first mode behavior across multiple messages

src/slack/monitor.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { isSlackRoomAllowedByPolicy } from "./monitor.js";
+import { isSlackRoomAllowedByPolicy, resolveSlackThreadTs } from "./monitor.js";
 
 describe("slack groupPolicy gating", () => {
   it("allows when policy is open", () => {
@@ -53,3 +53,83 @@ describe("slack groupPolicy gating", () => {
     ).toBe(false);
   });
 });
+
+describe("resolveSlackThreadTs", () => {
+  const threadTs = "1234567890.123456";
+
+  describe("replyToMode=off", () => {
+    it("returns baseThreadTs when in a thread", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "off",
+          baseThreadTs: threadTs,
+          hasReplied: false,
+        }),
+      ).toBe(threadTs);
+    });
+
+    it("returns baseThreadTs even after replies (stays in thread)", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "off",
+          baseThreadTs: threadTs,
+          hasReplied: true,
+        }),
+      ).toBe(threadTs);
+    });
+
+    it("returns undefined when not in a thread", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "off",
+          baseThreadTs: undefined,
+          hasReplied: false,
+        }),
+      ).toBeUndefined();
+    });
+  });
+
+  describe("replyToMode=first", () => {
+    it("returns baseThreadTs for first reply", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "first",
+          baseThreadTs: threadTs,
+          hasReplied: false,
+        }),
+      ).toBe(threadTs);
+    });
+
+    it("returns undefined for subsequent replies (goes to main channel)", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "first",
+          baseThreadTs: threadTs,
+          hasReplied: true,
+        }),
+      ).toBeUndefined();
+    });
+  });
+
+  describe("replyToMode=all", () => {
+    it("returns baseThreadTs for first reply", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "all",
+          baseThreadTs: threadTs,
+          hasReplied: false,
+        }),
+      ).toBe(threadTs);
+    });
+
+    it("returns baseThreadTs for subsequent replies (all go to thread)", () => {
+      expect(
+        resolveSlackThreadTs({
+          replyToMode: "all",
+          baseThreadTs: threadTs,
+          hasReplied: true,
+        }),
+      ).toBe(threadTs);
+    });
+  });
+});