feat(email): add skip_attachments option via config.yaml

* feat(email): add skip_attachments option via config.yaml

Adds a config.yaml-driven option to skip email attachments in the
gateway email adapter. Useful for malware protection and bandwidth
savings.

Configure in config.yaml:
  platforms:
    email:
      skip_attachments: true

Based on PR #1521 by @an420eth, changed from env var to config.yaml
(via PlatformConfig.extra) to match the project's config-first pattern.

* docs: document skip_attachments option for email adapter

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

@@ -118,6 +118,18 @@ Replies are sent via SMTP with proper email threading:
 
 The agent can send file attachments in replies. Include `MEDIA:/path/to/file` in the response and the file is attached to the outgoing email.
 
+### Skipping Attachments
+
+To ignore all incoming attachments (for malware protection or bandwidth savings), add to your `config.yaml`:
+
+```yaml
+platforms:
+  email:
+    skip_attachments: true
+```
+
+When enabled, attachment and inline parts are skipped before payload decoding. The email body text is still processed normally.
+
 ---
 
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