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.providers-whatsapp.ts

@@ -46,6 +46,7 @@ export const WhatsAppAccountSchema = z
         group: z.enum(["always", "mentions", "never"]).optional().default("mentions"),
       })
       .optional(),
+    debounceMs: z.number().int().nonnegative().optional().default(0),
   })
   .superRefine((value, ctx) => {
     if (value.dmPolicy !== "open") return;
@@ -101,6 +102,7 @@ export const WhatsAppConfigSchema = z
         group: z.enum(["always", "mentions", "never"]).optional().default("mentions"),
       })
       .optional(),
+    debounceMs: z.number().int().nonnegative().optional().default(0),
   })
   .superRefine((value, ctx) => {
     if (value.dmPolicy !== "open") return;