feat(rules): add BlocklistRule

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

internal/rules/blocklist.go

@@ -0,0 +1,26 @@
+package rules
+
+import "strings"
+
+// BlocklistRule rejects emails from configured senders.
+type BlocklistRule struct {
+	emails map[string]bool
+}
+
+// NewBlocklistRule creates a blocklist rule from a list of email addresses.
+func NewBlocklistRule(emails []string) *BlocklistRule {
+	m := make(map[string]bool, len(emails))
+	for _, e := range emails {
+		m[strings.ToLower(e)] = true
+	}
+	return &BlocklistRule{emails: m}
+}
+
+func (r *BlocklistRule) Name() string { return "blocklist" }
+
+func (r *BlocklistRule) Evaluate(ctx EmailContext) RuleResult {
+	if r.emails[strings.ToLower(ctx.Sender)] {
+		return Reject("blocked sender")
+	}
+	return Accept()
+}