security(helpers): filter __proto__ and constructor in deepCopy

[Goosterhof]

Summary

Closes Sapper M1 finding M5 — deepCopy prototype pollution (field report lives in the war-room meta-repo, not this one).

The bug

JSON.parse treats __proto__ and constructor as literal own enumerable keys, unlike object literals where __proto__ is a getter/setter on Object.prototype. That means:

```js
const payload = JSON.parse('{"proto": {"polluted": "yes"}}');
Object.keys(payload); // ["proto"] ← own enumerable
```

The previous deepCopy iterated with Object.keys and assigned via copiedObject[key] = .... When key === "__proto__", that assignment triggered the prototype setter, replacing the copy's prototype with an attacker-controlled object. A consumer checking inherited properties (e.g., if (obj.admin)) without Object.hasOwn would then be fooled.

The fix

One line added to the iteration:

```ts
for (const key of Object.keys(toCopy)) {
if (key === "proto" || key === "constructor") continue;
copiedObject[key] = deepCopy(...);
}
```

No legitimate caller supplies plain-object data with a literal `proto` own property — that only arises from untrusted input. `constructor` filtering is defense-in-depth (it never triggers prototype ops, but can shadow the real `constructor` on the copy, which is a known attack surface in some frameworks).

Tests

3 new cases in `prototype pollution resistance`:

1. Preserves prototype on `proto` injection — `Object.getPrototypeOf(copy)` remains `Object.prototype`, inherited `polluted` is undefined.
2. Drops literal `constructor` key — `Object.hasOwn(copy, "constructor")` is `false`; global `Object.prototype` is not polluted.
3. Preserves safe keys when dangerous keys are present — `safe` and `other` still copy correctly; `polluted` still inaccessible.

All 8 local gates pass: format, lint, build, typecheck, lint:pkg, 100% coverage, 100% mutation score on `deep-copy.ts` (23 mutants killed).

Release

Bumps `@script-development/fs-helpers` to `0.1.1`. On merge the publish workflow fires (triggered by `**/package.json` change on `main`), ships the patch to npm.

Test plan

• CI `check` job passes
• Ally review
• Merge triggers publish of 0.1.1 with sigstore provenance
• Spot-check npm: @script-development/fs-helpers@0.1.1 present

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