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

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

internal/rules/types.go

@@ -0,0 +1,41 @@
+package rules
+
+// EmailContext contains the data available to rules for evaluation.
+type EmailContext struct {
+	Sender    string
+	Subject   string
+	MessageID string
+}
+
+// RuleResult is the output of evaluating a single rule.
+type RuleResult struct {
+	Accepted bool
+	Reason   string
+	Metadata map[string]string
+}
+
+// Accept returns a passing RuleResult.
+func Accept() RuleResult {
+	return RuleResult{Accepted: true}
+}
+
+// Reject returns a failing RuleResult with a reason.
+func Reject(reason string) RuleResult {
+	return RuleResult{Accepted: false, Reason: reason}
+}
+
+// WithMetadata attaches a key-value pair to the result.
+func (r RuleResult) WithMetadata(key, value string) RuleResult {
+	if r.Metadata == nil {
+		r.Metadata = map[string]string{key: value}
+	} else {
+		r.Metadata[key] = value
+	}
+	return r
+}
+
+// Rule evaluates an email against operator-configurable policy.
+type Rule interface {
+	Name() string
+	Evaluate(ctx EmailContext) RuleResult
+}