feat(conformance): implement ReadOnly brand and reader generator

- Add ReadOnly<F> phantom brand to core-shared/conformance (compile-time
  enforcement that readers only wrap non-mutating use cases)
- Add isReadOnly runtime predicate for boot-time assertReaderPurity
- Scaffold pnpm turbo gen reader: creates integrations/readers/ with
  interface, implementation, test, barrel, and adds ./reader export
  subpath to package.json

turbo/generators/templates/reader/reader.interface.ts.hbs

@@ -0,0 +1,14 @@
+// packages/{{kebabCase feature}}/src/integrations/readers/{{kebabCase feature}}.reader.interface.ts
+
+/**
+ * Cross-feature read-only query contract for the {{kebabCase feature}} vertical.
+ * Consumers import this type from `@repo/{{kebabCase feature}}/reader`.
+ * Implementation is private — constructed by the feature's binder.
+ *
+ * Rules: Q0 (cross-feature domain queries only), Q1 (interface public,
+ * impl private), Q2 (read-only — wraps only mutates:false use cases).
+ */
+export interface I{{pascalCase feature}}Reader {
+  // Add methods as consumers need them. Each method must delegate to a
+  // use case declared mutates: false in feature.manifest.ts.
+}