fix: honor message tool channel for tool dedupe (#1053)

- Treat message tool `channel` as provider hint for dedupe/suppression.
- Prefer NO_REPLY after message tool sends to avoid duplicate replies.

Co-authored-by: Sash Catanzarite <1166151+thesash@users.noreply.github.com>

src/agents/system-prompt.test.ts

@@ -216,6 +216,7 @@ describe("buildAgentSystemPrompt", () => {
 
     expect(prompt).toContain("message: Send messages and channel actions");
     expect(prompt).toContain("### message tool");
+    expect(prompt).toContain("respond with ONLY: NO_REPLY");
   });
 
   it("includes runtime provider capabilities when present", () => {