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.
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