Scope the email guide to the top-level digest send files

Extend docs/guides/email.md paths: frontmatter (and its AGENTS.md index entry) to
include backend/digest_mailer.py and backend/smtp_send.py, so the guide auto-loads
when editing the outbound-digest send path — not just backend/email_integration/**.
Portability-checker: compliant.

docs/guides/email.md

@@ -1,6 +1,8 @@
 ---
 paths:
   - backend/email_integration/**
+  - backend/digest_mailer.py
+  - backend/smtp_send.py
 ---
 
 # Email capture & drafts (Gmail)