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>

docs/operations/adapter-remediation-ledger.md

@@ -314,8 +314,50 @@ Run on the landed tree. Only what actually passed is claimed as passing.
 | --- | --- | --- |
 | `tests/unit/ci-workflow-generation.test.ts` | 82 failed / 325 passed | Identical on the pre-change baseline (`git stash` comparison). The subprocess gates it spawns cannot run in this sandbox. |
 | `tests/unit/ci-artifact-contract.test.ts` | fails | Unchanged pre-existing sandbox, cgroup and `/tmp` permission behaviour. |
-| `tests/unit/removal-fixture.test.ts`, `tests/unit/supply-chain.test.ts`, `tests/integration/security-followup-archive.test.ts` | destructive | `scripts/lib/removal-fixture.ts:45` and `scripts/check-supply-chain-provider-fixtures.ts:62` symlink the real `node_modules` into a temp fixture root and then run `corepack pnpm` there with `CI=true`. pnpm purges the modules directory it does not recognise, **through the symlink**, deleting the repository's own dependencies mid-run. This is a pre-existing repository hazard, outside the 38 findings, and it is why a full `test:unit` run reports cascading failures. |
-| `tests/unit/security-followup.test.ts` | flaky under load | Passes in a fresh process; the process-group reaping assertion is timing sensitive. |
+| `tests/unit/security-followup.test.ts`, `tests/unit/provider-guardian-transaction.test.ts` | flaky under full-suite load | Both pass in a fresh process (49 passed). They spawn and reap process groups, so their timing assertions are load sensitive. |
+
+### Destructive fixture hazard — fixed
+
+Outside the 38 findings, and found while running the suites for them.
+
+Four sites linked the repository's installed dependencies into a throwaway
+fixture with a single directory symlink at `<fixture>/node_modules`:
+
+- `scripts/lib/removal-fixture.ts`
+- `scripts/check-supply-chain-provider-fixtures.ts`
+- `tests/integration/security-followup-archive.test.ts`
+- `tests/unit/ci-artifact-contract.test.ts`
+
+Each fixture then runs `pnpm` inside itself. 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 — a test suite uninstalling 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 during this work.
+
+`scripts/lib/fixture-node-modules.ts` replaces all four: `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. `tests/unit/fixture-node-modules.test.ts` performs the exact
+recursive delete pnpm performs and asserts the source tree survives, and
+`corepack pnpm check:adapter-inventory` fails on any reintroduction of the
+directory-symlink form.
+
+After the fix a full `tests/unit` + `tests/integration` run leaves the
+dependencies intact and its failures are confined to the two environmental
+files above plus the two flaky-under-load ones:
+
+| File | Failed | Attribution |
+| --- | ---: | --- |
+| `tests/unit/ci-workflow-generation.test.ts` | 82 | Identical on the pre-change baseline (`git stash` comparison). Its subprocess gates cannot run in this sandbox. |
+| `tests/unit/ci-artifact-contract.test.ts` | 19 | Unchanged pre-existing sandbox, cgroup and `/tmp` permission behaviour. |
+| `tests/unit/security-followup.test.ts` | 2 | Passes in isolation. |
+| `tests/unit/provider-guardian-transaction.test.ts` | 1 | Passes in isolation. |
+
+1619 passed / 1723 total, and `tests/unit/removal-fixture.test.ts`,
+`tests/unit/supply-chain.test.ts` and
+`tests/integration/security-followup-archive.test.ts` — the three that had to be
+excluded before — now pass in the full run.
 
 ## Rules for updating this ledger