feat: wire external rules pipeline into ingress and dispatch

- Add DispatchContext field to Task model for rule metadata storage
- Add BlocklistEmails config (BLOCKLIST_EMAILS env var)
- Wire rules.Pipeline into IMAPWatcher, replacing inline whitelist check
- Pass rule metadata through DispatchContext to OpenClaw dispatch
- Remove isWhitelisted method (now handled by WhitelistRule)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/database/models.go

@@ -24,6 +24,7 @@ type Task struct {
 	Sender          string
 	Subject         string
 	BodyPlain       string
+	DispatchContext string // JSON metadata from external rules pipeline.
 	AIResponse      string
 	Status          string `gorm:"index;not null"`
 	AttemptOpenClaw int    `gorm:"default:0"`