refactor(discord): clean autoThread context wiring (#856)

Build reply/session context once (no post-hoc ctx mutation) and type into the actual delivery target.

Thanks @davidguttman.

Co-authored-by: David Guttman <david@davidguttman.com>

src/discord/monitor/threading.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from "vitest";
+import { buildAgentSessionKey } from "../../routing/resolve-route.js";
+import { resolveDiscordAutoThreadContext } from "./threading.js";
+
+describe("resolveDiscordAutoThreadContext", () => {
+  it("returns null when no createdThreadId", () => {
+    expect(
+      resolveDiscordAutoThreadContext({
+        agentId: "agent",
+        channel: "discord",
+        messageChannelId: "parent",
+        createdThreadId: undefined,
+      }),
+    ).toBeNull();
+  });
+
+  it("re-keys session context to the created thread", () => {
+    const context = resolveDiscordAutoThreadContext({
+      agentId: "agent",
+      channel: "discord",
+      messageChannelId: "parent",
+      createdThreadId: "thread",
+    });
+    expect(context).not.toBeNull();
+    expect(context?.To).toBe("channel:thread");
+    expect(context?.From).toBe("group:thread");
+    expect(context?.OriginatingTo).toBe("channel:thread");
+    expect(context?.SessionKey).toBe(
+      buildAgentSessionKey({
+        agentId: "agent",
+        channel: "discord",
+        peer: { kind: "channel", id: "thread" },
+      }),
+    );
+    expect(context?.ParentSessionKey).toBe(
+      buildAgentSessionKey({
+        agentId: "agent",
+        channel: "discord",
+        peer: { kind: "channel", id: "parent" },
+      }),
+    );
+  });
+});
+