feat(util): extract dsh-atomic-write and migrate settings-local writes

writeFileAtomic: exclusive-create random-suffix temp + rename carrying the
caller-stated mode; settings-local persistSection now consumes it. The
credentials-local store shares it next.

packages/util/atomic-write/src/invariant.ts

@@ -0,0 +1,30 @@
+/**
+ * Package-owned invariant companion for `@deepseek-ai/dsh-atomic-write`.
+ * @module @deepseek-ai/dsh-atomic-write/invariant
+ */
+
+/* jscpd:ignore-start */
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-atomic-write'
+
+/** Cordis companion plugin name. */
+export const name = 'atomic-write-invariant'
+/** Service required before the companion can reserve package ownership. */
+export const inject = ['invariants']
+
+/**
+ * No runtime invariant: this pure filesystem primitive owns no event stream or mutable runtime
+ * data; its replacement contract is enforced by unit tests.
+ */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register this package's invariant companion.
+ * @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))
+/* jscpd:ignore-end */