Phase 7 follow-up: 1-month body retention and actions_log cap (AUD-75)

Implements the AGENTS.md §7 data rule: keep message bodies for one month, and
size-cap the append-only action log.

- internal/store/retention.go: Store.Retain clears messages.body_text older
  than the window (metadata stays) and deletes actions_log rows beyond the
  newest N, in one transaction.
- internal/config: EMAIL_AGENT_RETENTION_DAYS (30) and
  EMAIL_AGENT_ACTIONS_LOG_MAX_ROWS (50000).
- cmd/email-agent: bounded -retain flag for the n8n schedule.
- Tests for body clearing, metadata retention, log cap, and disabled policy.

.env.example

@@ -65,3 +65,9 @@ APPRISE_KEY=
 APPRISE_URLS=
 APPRISE_TOKEN=
 EMAIL_AGENT_SUMMARY_TO=samuelrolfe@gmail.com
+
+# --- Retention (AGENTS.md §7) -----------------------------------------------
+# `email-agent -retain` clears message bodies older than RETENTION_DAYS
+# (metadata stays) and caps the append-only actions_log at MAX_ROWS.
+EMAIL_AGENT_RETENTION_DAYS=30
+EMAIL_AGENT_ACTIONS_LOG_MAX_ROWS=50000