feat(companion): export platform shell templates for app starters

src/companion/index.ts

@@ -11,6 +11,7 @@ export {
 export { CompanionHeartbeatLoop } from './heartbeatLoop.js';
 export { createCompanionBootstrapManifest } from './bootstrapManifest.js';
 export { writeCompanionReleaseBundle } from './releaseBundle.js';
+export { writeCompanionShellTemplate } from './shellTemplate.js';
 
 export type {
   CompanionRuntimeClientOptions,
@@ -81,3 +82,8 @@ export type {
   WriteCompanionReleaseBundleInput,
   WriteCompanionReleaseBundleResult,
 } from './releaseBundle.js';
+export type {
+  CompanionShellTemplatePlatform,
+  WriteCompanionShellTemplateInput,
+  WriteCompanionShellTemplateResult,
+} from './shellTemplate.js';