feat: add template engine with {{variable}} substitution

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/template-engine.ts

@@ -0,0 +1,33 @@
+/**
+ * Simple {{variable}} substitution engine.
+ * Built-in variables: {{date}}, {{time}}, {{datetime}}.
+ * All other {{keys}} are resolved from the provided values map.
+ */
+export function renderTemplate(
+  template: string,
+  values: Record<string, string>,
+): string {
+  const now = new Date();
+  const builtins: Record<string, string> = {
+    date: now.toISOString().slice(0, 10),
+    time: now.toTimeString().slice(0, 5),
+    datetime: `${now.toISOString().slice(0, 10)} ${now.toTimeString().slice(0, 5)}`,
+  };
+
+  const allValues = { ...builtins, ...values };
+
+  return template.replace(/\{\{(\w[\w-]*)\}\}/g, (match, key: string) => {
+    return allValues[key] ?? match;
+  });
+}
+
+/**
+ * Sanitize a string for use as a file name.
+ * Removes characters illegal on Windows/macOS/Linux.
+ */
+export function sanitizeFileName(name: string): string {
+  return name
+    .replace(/[\\/:*?"<>|]/g, '-')
+    .replace(/\s+/g, ' ')
+    .trim();
+}