feat: support ignoring unauthorized gateway DMs (#1919)

Add unauthorized_dm_behavior config (pair|ignore) with global default
and per-platform override. WhatsApp can silently drop unknown DMs
instead of sending pairing codes.

Adapted config bridging to work with gw_data dict (pre-construction)
rather than config object. Dropped implementation plan document.

Co-authored-by: Frederico Ribeiro <fr@tecompanytea.com>

website/docs/user-guide/configuration.md

@@ -1090,6 +1090,21 @@ group_sessions_per_user: true  # true = per-user isolation in groups/channels, f
 
 For the behavior details and examples, see [Sessions](/docs/user-guide/sessions) and the [Discord guide](/docs/user-guide/messaging/discord).
 
+## Unauthorized DM Behavior
+
+Control what Hermes does when an unknown user sends a direct message:
+
+```yaml
+unauthorized_dm_behavior: pair
+
+whatsapp:
+  unauthorized_dm_behavior: ignore
+```
+
+- `pair` is the default. Hermes denies access, but replies with a one-time pairing code in DMs.
+- `ignore` silently drops unauthorized DMs.
+- Platform sections override the global default, so you can keep pairing enabled broadly while making one platform quieter.
+
 ## Quick Commands
 
 Define custom commands that run shell commands without invoking the LLM — zero token usage, instant execution. Especially useful from messaging platforms (Telegram, Discord, etc.) for quick server checks or utility scripts.