fix(headless): dsh run is a direct core front door

packages/core/agent-default-model/src/invariant.ts

@@ -0,0 +1,30 @@
+/**
+ * Package-owned invariant companion for the default Agent model selection.
+ *
+ * The service owns no independent event relationship: settings registration
+ * already validates every mutable value before `currentSelection()` can observe it.
+ * The empty installer keeps that absence explicit in composed invariant sets.
+ *
+ * @module @deepseek-ai/dsh-agent-default-model/invariant
+ */
+
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-agent-default-model'
+
+/** Cordis companion plugin name. */
+export const name = 'agent-default-model-invariant'
+/** Services required before the companion can register. */
+export const inject = ['invariants']
+
+/** No runtime invariant: settings validation owns the only mutable-value relationship. */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register the intentionally empty invariant contribution.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))