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-20-cutover.md

@@ -1,7 +1,8 @@
 # Task 20 — Cutover and cleanup
 
-**Agent**: `astro-architect`, **with a human watching** · **Model**: MiniMax-M3 assisting
-**Depends on**: all · **Worktree**: `.agents/scripts/worktree.sh start 20 cutover`
+**Agent**: `astro-architect`, **with a human watching** · **Model**: MiniMax-M3
+assisting **Depends on**: all · **Worktree**:
+`.agents/scripts/worktree.sh start 20 cutover`
 
 This task touches production publishing. Do not run it unattended.
 
@@ -18,9 +19,9 @@ describe reality.
    `app.js`, `styles.css`, `responsive.css`, `chapters.css`, `landing.css`,
    `rules/app.js`, `rules/styles.css`, `skills/app.js`, `skills/styles.css`,
    `skills-review/*.js`, `skills-review/*.css`, and the ten old `index.html`
-   files. `git rm`, one commit, reviewable.
-   **Keep**: `hands-on/**` (now under `public/`), `submitted-skills/**`,
-   `skill-reviews/**`, `docs/**`, `vote-service/**`.
+   files. `git rm`, one commit, reviewable. **Keep**: `hands-on/**` (now under
+   `public/`), `submitted-skills/**`, `skill-reviews/**`, `docs/**`,
+   `vote-service/**`.
 3. **Publish** via the mechanism chosen in task 01.
 4. **Verify on the real host** with a cache-buster:
    ```bash
@@ -53,6 +54,6 @@ git rev-parse origin/pages   # write it down
 - [ ] Ten routes 200 on the real host with a fresh cache-buster
 - [ ] Vote widget works end-to-end from the published origin (CORS is
       origin-sensitive — `ALLOWED_ORIGIN` must still match)
-- [ ] Old files deleted; `npm run gate` green
+- [ ] Old files deleted; `pnpm run gate` green
 - [ ] Docs match reality
 - [ ] Previous `pages` SHA recorded for rollback