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.

AGENTS.md

@@ -61,7 +61,7 @@ Subsystem rules live in `docs/guides/` and lazy-load in Claude Code via `.claude
 - **Thesis logic** (`backend/thesis_*.py`, `backend/mcp/architect_*.py`) → `docs/guides/thesis.md`
 - **Redaction or any MCP/Claude path** (`backend/redaction/`, `backend/mcp/`) → `docs/guides/redaction.md`
 - **Ingest / retrieval** (`backend/ingest/`) → `docs/guides/spark-ingest.md`
-- **Email capture / drafts** (`backend/email_integration/`) → `docs/guides/email.md`
+- **Email capture / drafts + digest send** (`backend/email_integration/`, `backend/digest_mailer.py`, `backend/smtp_send.py`) → `docs/guides/email.md`
 - **Building or deploying the s9pk** (`start9/`) → `docs/guides/packaging.md`
 
 ## Conventions

docs/guides/email.md

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