fix: stop test fixtures from deleting the repository's dependencies

Four fixtures linked the installed dependencies into a throwaway root with a
single directory symlink at <fixture>/node_modules, then ran pnpm inside that
root. pnpm does not recognise the modules directory it finds there and purges
it; with CI=true it does so without a prompt. The purge followed the symlink and
deleted the repository's own node_modules mid-run, so a test suite uninstalled
the workspace it was running in. That is what produced the cascading,
file-unrelated failures a full test:unit run reported, and it happened twice
while running the suites for the adapter re-review.

scripts/lib/fixture-node-modules.ts replaces all four sites: node_modules is a
real directory whose entries are individual symlinks, so a recursive delete
unlinks the fixture's own links instead of walking through one link into the
shared tree. Resolution is unchanged.

tests/unit/fixture-node-modules.test.ts performs the exact recursive delete pnpm
performs and asserts the source tree survives, and check:adapter-inventory now
fails on any reintroduction of the directory-symlink form — verified by putting
the old line back and watching the gate reject it.

A full tests/unit + tests/integration run now leaves the dependencies intact.
removal-fixture, supply-chain and security-followup-archive, the three suites
that had to be excluded before, pass in that run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/lib/removal-fixture.ts

@@ -5,7 +5,6 @@ import {
   readFile,
   readdir,
   rm,
-  symlink,
   writeFile,
 } from "node:fs/promises";
 import path from "node:path";
@@ -14,6 +13,7 @@ import {
   loadCiGateContract,
   parseCiGateContract,
 } from "../contracts/ci-gates.ts";
+import { linkFixtureNodeModules } from "./fixture-node-modules.ts";
 import { generateCiWorkflow } from "../generate-ci-workflow.ts";
 
 export const REMOVAL_FIXTURE_COPY_TARGETS = Object.freeze([
@@ -42,7 +42,7 @@ export async function prepareRemovalFixture(
   for (const target of copyTargets) {
     await cp(target, path.join(root, target), { recursive: true });
   }
-  await symlink(path.resolve("node_modules"), path.join(root, "node_modules"), "dir");
+  await linkFixtureNodeModules(root);
 }
 
 export function runRemovalFixturePnpm(