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

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

internal/rules/pipeline.go

@@ -0,0 +1,30 @@
+package rules
+
+// Pipeline evaluates a sequence of rules, stopping on first rejection.
+type Pipeline struct {
+	rules []Rule
+}
+
+// NewPipeline creates a rules pipeline from the given rules.
+func NewPipeline(rules []Rule) *Pipeline {
+	return &Pipeline{rules: rules}
+}
+
+// Evaluate runs all rules in order. Returns the first rejection or a merged accept.
+func (p *Pipeline) Evaluate(ctx EmailContext) RuleResult {
+	merged := Accept()
+	for _, r := range p.rules {
+		result := r.Evaluate(ctx)
+		if !result.Accepted {
+			return result
+		}
+		for k, v := range result.Metadata {
+			if merged.Metadata == nil {
+				merged.Metadata = map[string]string{k: v}
+			} else {
+				merged.Metadata[k] = v
+			}
+		}
+	}
+	return merged
+}