Add shared replacement method

packages/shared/src/index.ts

@@ -1,2 +1,3 @@
 export * from './schemas/index.js';
 export * from './operators.js';
+export * from './template.js';

packages/shared/src/template.ts

@@ -0,0 +1,38 @@
+/**
+ * Render email template by replacing variables
+ * Supports {{variable}} and {{variable ?? defaultValue}} syntax
+ * Also supports nested access like {{data.firstName}}
+ *
+ * Example:
+ * renderTemplate('Hello {{name}}!', { name: 'World' }) -> 'Hello World!'
+ * renderTemplate('Hello {{data.name}}!', { data: { name: 'World' } }) -> 'Hello World!'
+ * renderTemplate('Hello {{name ?? Guest}}!', {}) -> 'Hello Guest!'
+ */
+export function renderTemplate(template: string, variables: Record<string, unknown>): string {
+  return template.replace(/\{\{(.*?)\}\}/g, (match, key) => {
+    const [mainKey, defaultValue] = key.split('??').map((s: string) => s.trim());
+
+    // Handle nested property access (e.g., data.firstName)
+    const getValue = (obj: Record<string, unknown>, path: string): unknown => {
+      return path.split('.').reduce((current: Record<string, unknown> | unknown, key) => {
+        if (current && typeof current === 'object' && !Array.isArray(current)) {
+          return (current as Record<string, unknown>)[key];
+        }
+        return undefined;
+      }, obj);
+    };
+
+    // Try multiple lookup strategies
+    const value =
+      getValue(variables, mainKey) || // Try as nested path (e.g., data.firstName)
+      variables[mainKey] || // Try as top-level property
+      (variables.data as Record<string, unknown>)?.[mainKey]; // Try in data object
+
+    // Handle array values (for lists)
+    if (Array.isArray(value)) {
+      return value.map((e: string) => `<li>${e}</li>`).join('\n');
+    }
+
+    return value ?? defaultValue ?? '';
+  });
+}