fix(gateway): config.yaml path for WhatsApp/Weixin text-batch delays

Convert the salvaged text-debounce delays from HERMES_* env vars to
config.yaml (gateway.platforms.<name>.extra.text_batch_delay_seconds /
text_batch_split_delay_seconds), per the '.env is for secrets only'
policy. Adds a finite/non-negative guard so bad YAML values fall back to
the defaults instead of crashing asyncio.sleep().

- whatsapp.py / weixin.py: read delays via _coerce_float_extra(config.extra)
- update Weixin content-dedup regression test for the deferred dispatch path
- add text-debounce coverage (whatsapp + weixin): defaults, config override,
  bad-value fallback, env-var-ignored, burst-collapse, lone-message
- docs: WhatsApp + Weixin config keys

website/docs/user-guide/messaging/whatsapp.md

@@ -201,6 +201,22 @@ Code blocks and inline code are preserved as-is since WhatsApp supports triple-b
 
 When the agent calls tools (web search, file operations, etc.), WhatsApp displays real-time progress indicators showing which tool is running. This is enabled by default — no configuration needed.
 
+### Message Batching (Debounce)
+
+WhatsApp delivers each message individually, so a rapid burst (forwarded batches, paste-splits, multi-line text) would otherwise trigger a separate agent invocation per fragment — wasting tokens and producing several disjointed replies. The adapter buffers successive text messages from the same chat and dispatches them as one combined request after a short quiet period (default **5s**, extended to **10s** for very long fragments). Tune via `config.yaml`:
+
+```yaml
+# ~/.hermes/config.yaml
+gateway:
+  platforms:
+    whatsapp:
+      extra:
+        text_batch_delay_seconds: 5.0         # quiet period before flushing a batch
+        text_batch_split_delay_seconds: 10.0  # extended delay near the split threshold
+```
+
+Set `text_batch_delay_seconds: 0` to dispatch each message immediately (disables batching).
+
 ---
 
 ## Troubleshooting