feat(rules): add Rule interface, Pipeline, and WhitelistRule

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

internal/rules/whitelist.go

@@ -0,0 +1,30 @@
+package rules
+
+import "strings"
+
+// WhitelistRule accepts emails from configured senders.
+// An empty list accepts all senders.
+type WhitelistRule struct {
+	emails map[string]bool
+}
+
+// NewWhitelistRule creates a whitelist rule from a list of email addresses.
+func NewWhitelistRule(emails []string) *WhitelistRule {
+	m := make(map[string]bool, len(emails))
+	for _, e := range emails {
+		m[strings.ToLower(e)] = true
+	}
+	return &WhitelistRule{emails: m}
+}
+
+func (r *WhitelistRule) Name() string { return "whitelist" }
+
+func (r *WhitelistRule) Evaluate(ctx EmailContext) RuleResult {
+	if len(r.emails) == 0 {
+		return Accept()
+	}
+	if r.emails[strings.ToLower(ctx.Sender)] {
+		return Accept()
+	}
+	return Reject("not whitelisted")
+}