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/rules/whitelist.go

@@ -27,4 +27,4 @@ func (r *WhitelistRule) Evaluate(ctx EmailContext) RuleResult {
 		return Accept()
 	}
 	return Reject("not whitelisted")
-}
+}