feat(whatsapp): add debounceMs for batching rapid messages (#971)

* feat(whatsapp): add debounceMs for batching rapid messages

Add a `debounceMs` configuration option to WhatsApp channel settings
that batches rapid consecutive messages from the same sender into a
single response. This prevents triggering separate agent runs for
each message when a user sends multiple short messages in quick
succession (e.g., "Hey!", "how are you?", "I was wondering...").

Changes:
- Add `debounceMs` config to WhatsAppConfig and WhatsAppAccountConfig
- Implement message buffering in `monitorWebInbox` with:
  - Map-based buffer keyed by sender (DM) or chat ID (groups)
  - Debounce timer that resets on each new message
  - Message combination with newline separator
  - Single message optimization (no modification if only one message)
- Wire `debounceMs` through account resolution and monitor tuning
- Add UI hints and schema documentation

Usage example:
{
  "channels": {
    "whatsapp": {
      "debounceMs": 5000  // 5 second window
    }
  }
}

Default behavior: `debounceMs: 0` (disabled by default)

Verified: All existing tests pass (3204 tests), TypeScript compilation
succeeds with no errors.

Implemented with assistance from AI coding tools.

Closes #967

* chore: wip inbound debounce

* fix: debounce inbound messages across channels (#971) (thanks @juanpablodlc)

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/auto-reply/inbound-debounce.test.ts

@@ -0,0 +1,48 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { createInboundDebouncer } from "./inbound-debounce.js";
+
+describe("createInboundDebouncer", () => {
+  it("debounces and combines items", async () => {
+    vi.useFakeTimers();
+    const calls: Array<string[]> = [];
+
+    const debouncer = createInboundDebouncer<{ key: string; id: string }>({
+      debounceMs: 10,
+      buildKey: (item) => item.key,
+      onFlush: async (items) => {
+        calls.push(items.map((entry) => entry.id));
+      },
+    });
+
+    await debouncer.enqueue({ key: "a", id: "1" });
+    await debouncer.enqueue({ key: "a", id: "2" });
+
+    expect(calls).toEqual([]);
+    await vi.advanceTimersByTimeAsync(10);
+    expect(calls).toEqual([["1", "2"]]);
+
+    vi.useRealTimers();
+  });
+
+  it("flushes buffered items before non-debounced item", async () => {
+    vi.useFakeTimers();
+    const calls: Array<string[]> = [];
+
+    const debouncer = createInboundDebouncer<{ key: string; id: string; debounce: boolean }>({
+      debounceMs: 50,
+      buildKey: (item) => item.key,
+      shouldDebounce: (item) => item.debounce,
+      onFlush: async (items) => {
+        calls.push(items.map((entry) => entry.id));
+      },
+    });
+
+    await debouncer.enqueue({ key: "a", id: "1", debounce: true });
+    await debouncer.enqueue({ key: "a", id: "2", debounce: false });
+
+    expect(calls).toEqual([["1"], ["2"]]);
+
+    vi.useRealTimers();
+  });
+});