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.

internal/config/config.go

@@ -60,6 +60,11 @@ const (
 	// fans out to email/NTFY/Telegram. SUMMARY_TO is the summary recipient.
 	DefaultAppriseURL = "http://192.168.20.35:8210"
 	DefaultSummaryTo  = "samuelrolfe@gmail.com"
+
+	// Retention defaults (AGENTS.md §7): keep message bodies for one month, and
+	// cap the append-only action log. The retention job is the CLI `-retain`.
+	DefaultRetentionDays     = 30
+	DefaultActionsLogMaxRows = 50000
 )
 
 // Config is the fully resolved runtime configuration.
@@ -153,6 +158,14 @@ type Config struct {
 	AppriseToken string
 	// SummaryTo is the recipient address for the daily summary email.
 	SummaryTo string
+
+	// RetentionDays is the message-body window in days (AGENTS.md §7: one
+	// month). Bodies of messages received more than this many days ago are
+	// cleared by the `-retain` job; metadata is kept.
+	RetentionDays int
+	// ActionsLogMaxRows caps the append-only actions_log. The oldest rows beyond
+	// this count are deleted by the `-retain` job.
+	ActionsLogMaxRows int
 }
 
 // Load reads configuration from the environment, applying defaults.
@@ -197,6 +210,8 @@ func Load() (*Config, error) {
 	c.IMAPMaxMessages = getenvDefaultInt("EMAIL_AGENT_IMAP_MAX_MESSAGES", DefaultIMAPMaxMessages)
 	c.ClassifyConcurrency = getenvDefaultInt("EMAIL_AGENT_CLASSIFY_CONCURRENCY", DefaultClassifyConcurrency)
 	c.ClassifyMaxMessages = getenvDefaultInt("EMAIL_AGENT_CLASSIFY_MAX_MESSAGES", DefaultClassifyMaxMessages)
+	c.RetentionDays = getenvDefaultInt("EMAIL_AGENT_RETENTION_DAYS", DefaultRetentionDays)
+	c.ActionsLogMaxRows = getenvDefaultInt("EMAIL_AGENT_ACTIONS_LOG_MAX_ROWS", DefaultActionsLogMaxRows)
 
 	port, err := resolvePort()
 	if err != nil {