feat(mosaic): migrate install wizard from v0 to v1

Complete port of the ~2000 LOC mosaic install wizard from v0. Migrates
all 27 source files across the prompter, config, platform, template,
runtime, skills, and stages subsystems to v1 ESM/NodeNext conventions.

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

packages/mosaic/src/template/engine.ts

@@ -0,0 +1,23 @@
+export interface TemplateVars {
+  [key: string]: string;
+}
+
+/**
+ * Replaces {{PLACEHOLDER}} tokens with provided values.
+ * Does NOT expand ${ENV_VAR} syntax — those pass through for shell resolution.
+ */
+export function renderTemplate(
+  template: string,
+  vars: TemplateVars,
+  options: { strict?: boolean } = {},
+): string {
+  return template.replace(/\{\{([A-Z_][A-Z0-9_]*)\}\}/g, (match, varName: string) => {
+    if (varName in vars) {
+      return vars[varName] ?? '';
+    }
+    if (options.strict) {
+      throw new Error(`Template variable not provided: {{${varName}}}`);
+    }
+    return '';
+  });
+}