diff --git a/.github/workflows/pr-lockfile-lint.yml b/.github/workflows/pr-lockfile-lint.yml
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+name: Lockfile Lint
+
+# Prevents lockfile injection attacks by validating that every `resolved`
+# URL in yarn.lock uses HTTPS and points to a trusted registry host
+# (registry.yarnpkg.com). See:
+# https://github.com/lirantal/lockfile-lint
+
+on:
+  pull_request:
+    paths:
+      - 'yarn.lock'
+
+permissions:
+  contents: read
+
+jobs:
+  lockfile-lint:
+    name: Validate yarn.lock
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+        with:
+          persist-credentials: false
+          sparse-checkout: |
+            yarn.lock
+          sparse-checkout-cone-mode: false
+
+      - name: Setup Node.js
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f  # v6.3.0
+        with:
+          node-version: '22'
+
+      - name: Run lockfile-lint
+        run: |
+          npx --yes lockfile-lint@5.0.0 \
+            --path yarn.lock \
+            --type yarn \
+            --allowed-hosts yarn \
+            --validate-https
diff --git a/PLAN.md b/PLAN.md
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+# Plan: npm Supply-Chain Security — Lockfile Lint
+
+## Goal
+
+Add a PR check that validates `yarn.lock` against trusted registries,
+preventing lockfile injection attacks (modified `resolved` URLs pointing
+to attacker-controlled hosts).
+
+## Scope decisions
+
+| Decision | Choice | Rationale |
+|----------|--------|-----------|
+| Scope | Lockfile-lint only | See "Cooldown abandoned" below |
+| Dependabot | Skip — handled at org level | Already configured organization-wide |
+| Enforcement | PR check (GitHub Actions) | Cannot be bypassed; no local friction |
+| Workflow style | Dedicated `pr-lockfile-lint.yml` | Separate concern, separate trigger path |
+| lockfile-lint install | `npx` with pinned version | Zero new dependencies in `package.json` |
+| Pinned version | `lockfile-lint@5.0.0` | Latest stable, published with provenance via GitHub Actions OIDC |
+
+## Cooldown abandoned (originally Step 1)
+
+The original plan included adding `.npmrc` with `min-release-age=3`.
+Dropped after verification revealed two independent reasons it provides
+zero runtime protection to this repo today:
+
+1. **Yarn Classic v1 ignores the setting.** Yarn Classic was frozen in
+   early 2022; npm CLI 11.10.0 added `min-release-age` in late 2025.
+   Yarn Classic reads `.npmrc` only for registry URL and auth tokens,
+   not arbitrary npm resolution config. Verified empirically — `yarn
+   install --frozen-lockfile` runs cleanly with `.npmrc` present and
+   silently ignores the key.
+
+2. **Node 22.x bundles npm 10.9.7** (verified from `deps/npm/package.json`
+   on the `v22.x` branch of `nodejs/node`). The one workflow in this repo
+   that uses `npm install` (`auto-label.yml`) pins `node-version: '22'`,
+   so even if it did use an `.npmrc`, the bundled npm is too old to
+   understand `min-release-age`. Node 24.x ships npm 11.11.0 and would
+   work, but upgrading `auto-label.yml` is out of scope for this PR.
+
+Real cooldown protection for this repo requires migrating away from
+Yarn Classic. Tracked separately — see "Follow-up: pnpm migration".
+
+## Steps
+
+### Step 1: Create `pr-lockfile-lint.yml` workflow
+
+Create `.github/workflows/pr-lockfile-lint.yml` that:
+
+- Triggers on PRs that modify `yarn.lock`
+- Checks out the repo (no auth, shallow)
+- Runs `npx --yes lockfile-lint@5.0.0` against `yarn.lock`
+- Validates: all resolved URLs use HTTPS and resolve to
+  `registry.yarnpkg.com` (the `yarn` alias)
+- Fails the check if any URL is non-HTTPS or points to an untrusted host
+
+### Step 2: Verify
+
+- Baseline check: `grep 'resolved ' yarn.lock | grep -v 'https://registry.yarnpkg.com/'`
+  currently returns zero matches (802 entries, all clean). The first
+  run of the check must pass.
+- `actionlint` validates workflow syntax.
+- Open this PR; confirm the `Lockfile Lint` check appears and passes.
+
+## Follow-up: pnpm migration (separate PR)
+
+Real cooldown protection — plus several additional security benefits —
+requires migrating from Yarn Classic to pnpm. Tracked separately.
+Summary of what pnpm provides that Yarn Classic does not:
+
+- **First-class `minimumReleaseAge` cooldown** with exclude list support
+- **Post-install scripts disabled by default** (explicit allowlist via
+  `onlyBuiltDependencies`)
+- **`trustPolicy: no-downgrade`** — detects when a package's publish-time
+  trust level decreases (e.g., previously via GitHub Actions OIDC,
+  now without provenance)
+- **Stricter peer dependency resolution** (catches latent issues)
+- **Faster installs** via content-addressable store
+- **Drop-in compatible** with this PR's `lockfile-lint` check
+
+Migration scope for docs-v2 (single-package repo, no workspaces):
+
+- 14 workflow files: `yarn install --frozen-lockfile` → `pnpm install --frozen-lockfile`
+- Lockfile: `yarn.lock` → `pnpm-lock.yaml` (regenerated fresh)
+- New config: `pnpm-workspace.yaml` (optional but needed for cooldown/trust config)
+- Developer onboarding: `corepack enable` (built into Node 16+)
+- Risk: stricter peer deps may surface latent issues; mitigated by
+  running full test suite before merge
+
+## Out of scope (future work)
+
+### Medium priority
+
+- **Add `lint:lockfile` to lefthook pre-push** — local enforcement as
+  belt-and-suspenders alongside the PR check. Skippable with
+  `LEFTHOOK=0`, so lower priority than CI.
+- **Pin version ranges** — replace `^` and `>=` with exact versions for
+  production dependencies in `package.json`. High churn, moderate risk
+  reduction.
+
+### Low priority
+
+- **Install npq or sfw globally** — document recommended developer
+  machine hardening for ad-hoc `npm install` commands.
+- **Document the postinstall exemption** — explain in CONTRIBUTING or
+  CLAUDE.md why `--ignore-scripts` cannot be used globally (the repo's
+  `postinstall` script sets up the `docs` CLI).