build: migrate from npm to pnpm

Ten git worktrees each carried their own 225 MB node_modules (1.1 GB across
five) and paid 11s per `npm ci`. pnpm hardlinks from a shared store: the same
five worktrees cost ~250 MB total, and a fresh install is 4s.

What changed beyond the mechanical rename:

- `overrides` moved to `pnpm-workspace.yaml`. pnpm 11 does not read the `pnpm`
  field in package.json *or* npm's top-level `overrides`, and it fails silently
  — the vite/defu/language-server pins would have quietly stopped applying.
- Build scripts are blocked by default in pnpm; esbuild and sharp are allowed
  explicitly via `allowBuilds` (renamed from `onlyBuiltDependencies` in 11).
- `packageManager` + `engines` pin the toolchain.
- gate.sh rejects a package-lock.json/yarn.lock/bun.lock outright, so an agent
  running `npm install` out of habit fails loudly instead of building a second,
  divergent dependency tree.
- CI bootstraps pnpm with `npm install --global pnpm@11.25.0` rather than
  corepack (unbundled as of Node 25) or pnpm/action-setup (this self-hosted
  act-runner has never run a job; fetching a third-party action is not
  something to discover on the first one).

Two pre-existing CI bugs fixed while in the file:

- the gate installed with `npm install --package-lock=false`, which discarded
  the lockfile the previous session had just fixed.
- the visual-regression step imported `playwright`, which is not a dependency,
  and `visual-regression.mjs` has no compare mode anyway — in CI it overwrote
  its own baselines and passed unconditionally. Removed with a comment; it
  comes back when it can diff.

The `publish` job is now manual (`workflow_dispatch`). During the migration
dist/ holds three HTML files against the live pages branch's ten, so publishing
on every push to main would take the site down to a stub. Restore at task 20.

HANDOVER.md's incident log still says npm where it describes what happened at
the time; that is history, not a missed rename.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plans/astro-refactor/task-16-page-review-desk.md

@@ -1,14 +1,14 @@
 # Task 16 — Review desk
 
 **Agent**: `page-migrator` · **Model**: **Codex** — highest defect risk
-**Depends on**: 06, 11, 13 · **Parallel with**: 15
-**Worktree**: `.agents/scripts/worktree.sh start 16 page-review-desk`
+**Depends on**: 06, 11, 13 · **Parallel with**: 15 **Worktree**:
+`.agents/scripts/worktree.sh start 16 page-review-desk`
 
 ## Goal
 
-`/skills-review/` → Astro. The most interactive page: search, filtering,
-lazy file fetching, markdown rendering, a client-side diff view, six URL params,
-and a live API call to `vote-service/`.
+`/skills-review/` → Astro. The most interactive page: search, filtering, lazy
+file fetching, markdown rendering, a client-side diff view, six URL params, and
+a live API call to `vote-service/`.
 
 ## The six query params are a public contract
 
@@ -44,7 +44,8 @@ Interaction tokens: `from './catalog.js'`, `from './files.js'`, `renderList`,
 `markdownMarkup`, `syncUrl`, `selectFromUrl`, `URLSearchParams`,
 `navigator.clipboard`, `document.execCommand`.
 
-Plus the catalog count: `id:'` occurrences across both catalogs **must equal 24**.
+Plus the catalog count: `id:'` occurrences across both catalogs **must equal
+24**.
 
 Same rule as task 15 — re-point with task 19, never delete.
 
@@ -60,4 +61,4 @@ needs raw source text**, not rendered HTML. Keep both available.
 - [ ] Search, filter, file tabs, preview, change lens, download, copy all work
 - [ ] Vote widget reaches the live API; CORS preflight succeeds
 - [ ] Snapshot diff empty; screenshots match; checklist complete
-- [ ] `npm run gate` green
+- [ ] `pnpm run gate` green