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>

.agents/checklists/before-component.md

@@ -1,6 +1,7 @@
 # Checklist: before you call a component done
 
-- [ ] It appears (or will appear) in **three** places, or has a name a person says out loud
+- [ ] It appears (or will appear) in **three** places, or has a name a person
+      says out loud
 - [ ] Lives in the right folder: `primitives/`, `blocks/`, or `islands/`
 - [ ] Typed `interface Props`; every field intentional; no `any`
 - [ ] **No raw hex, px font sizes, or ad-hoc breakpoints** — tokens only
@@ -8,8 +9,10 @@
 - [ ] Markup under ~120 lines
 - [ ] Native elements: `<button>` for actions, `<a>` for navigation
 - [ ] Keyboard: tab to it, operate with Enter/Space, focus ring visible
-- [ ] Interactive state exposed via ARIA (`aria-pressed`, `aria-current`), not just colour
-- [ ] Any motion respects `prefers-reduced-motion` and animates only `transform`/`opacity`
+- [ ] Interactive state exposed via ARIA (`aria-pressed`, `aria-current`), not
+      just colour
+- [ ] Any motion respects `prefers-reduced-motion` and animates only
+      `transform`/`opacity`
 - [ ] Renders correctly at 560 / 800 / 1100 / 1600 px
 - [ ] Both `en` and `pt` strings present; no hard-coded copy
-- [ ] `npm run verify` green with no assertion deleted
+- [ ] `pnpm run verify` green with no assertion deleted