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

.agents/checklists/before-merge.md

@@ -1,9 +1,11 @@
 # Checklist: before you merge a task branch
 
 - [ ] Rebased on current `origin/main`, conflicts resolved in the worktree
-- [ ] `npm run verify` and `node scripts/audit-ui.mjs` both green
-- [ ] **Assertion count in `verify.mjs` did not fall** (`grep -c 'throw new Error' scripts/verify.mjs`)
-- [ ] Only files in your task's scope changed — `git diff --stat origin/main` matches the brief
+- [ ] `pnpm run verify` and `node scripts/audit-ui.mjs` both green
+- [ ] **Assertion count in `verify.mjs` did not fall**
+      (`grep -c 'throw new Error' scripts/verify.mjs`)
+- [ ] Only files in your task's scope changed — `git diff --stat origin/main`
+      matches the brief
 - [ ] No contended file edited unless you own it (see `rules/git-worktrees.md`)
 - [ ] No `.serena/`, `__pycache__/`, `dist/`, or scratch files staged
 - [ ] Commit message: what, why, and what you deliberately did not do

.agents/checklists/before-page.md

@@ -1,14 +1,20 @@
 # Checklist: before you call a page migrated
 
 - [ ] URL identical to the old one, **trailing slash included**
-- [ ] Any query params the old page honoured still work (`?author=`, `?skill=`, `?view=`, `?file=`, `?compare=`, `?render=`)
-- [ ] Rendered text content diffs clean against the pre-migration snapshot — or every difference is listed and justified
-- [ ] Same number of JS bytes or fewer; a previously-zero-JS page still ships zero
-- [ ] `<title>`, `<meta name="description">`, and `<meta name="viewport">` preserved
+- [ ] Any query params the old page honoured still work (`?author=`, `?skill=`,
+      `?view=`, `?file=`, `?compare=`, `?render=`)
+- [ ] Rendered text content diffs clean against the pre-migration snapshot — or
+      every difference is listed and justified
+- [ ] Same number of JS bytes or fewer; a previously-zero-JS page still ships
+      zero
+- [ ] `<title>`, `<meta name="description">`, and `<meta name="viewport">`
+      preserved
 - [ ] One `<h1>`; heading levels do not skip
-- [ ] Every ARIA attribute from the original survived (`verify.mjs` asserts several)
-- [ ] No external `<script>`, `<link>`, `@import`, or `url()` — `node scripts/audit-ui.mjs` green
+- [ ] Every ARIA attribute from the original survived (`verify.mjs` asserts
+      several)
+- [ ] No external `<script>`, `<link>`, `@import`, or `url()` —
+      `node scripts/audit-ui.mjs` green
 - [ ] Internal links go through `BASE_URL`, not hand-written absolute paths
 - [ ] Screenshots at 560 / 800 / 1100 / 1600 px compared against the old page
 - [ ] `<html lang>` correct and still switching with the language toggle
-- [ ] `npm run verify` green with no assertion deleted
+- [ ] `pnpm run verify` green with no assertion deleted