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/config/zod-schema.core.ts

@@ -188,6 +188,19 @@ export const QueueModeBySurfaceSchema = z
   })
   .optional();
 
+export const DebounceMsBySurfaceSchema = z
+  .object({
+    whatsapp: z.number().int().nonnegative().optional(),
+    telegram: z.number().int().nonnegative().optional(),
+    discord: z.number().int().nonnegative().optional(),
+    slack: z.number().int().nonnegative().optional(),
+    signal: z.number().int().nonnegative().optional(),
+    imessage: z.number().int().nonnegative().optional(),
+    msteams: z.number().int().nonnegative().optional(),
+    webchat: z.number().int().nonnegative().optional(),
+  })
+  .optional();
+
 export const QueueSchema = z
   .object({
     mode: QueueModeSchema.optional(),
@@ -198,6 +211,13 @@ export const QueueSchema = z
   })
   .optional();
 
+export const InboundDebounceSchema = z
+  .object({
+    debounceMs: z.number().int().nonnegative().optional(),
+    byChannel: DebounceMsBySurfaceSchema,
+  })
+  .optional();
+
 export const TranscribeAudioSchema = z
   .object({
     command: z.array(z.string()).superRefine((value, ctx) => {