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-18-motion.md

@@ -1,8 +1,8 @@
 # Task 18 — Motion pass
 
 **Agent**: `motion-designer` · **Model**: Gemini (perceptual judgement)
-**Depends on**: 15, 16 · **Parallel with**: 19
-**Worktree**: `.agents/scripts/worktree.sh start 18 motion`
+**Depends on**: 15, 16 · **Parallel with**: 19 **Worktree**:
+`.agents/scripts/worktree.sh start 18 motion`
 
 ## Goal
 
@@ -13,7 +13,8 @@ Audit the motion that exists, add only motion that earns its place, and make
 
 The audit is worth more than the additions.
 
-1. Inventory every `transition`, `animation`, `@keyframes`, `transform` in `src/`.
+1. Inventory every `transition`, `animation`, `@keyframes`, `transform` in
+   `src/`.
 2. Flag anything animating a layout property (`width`, `height`, `top`, `left`,
    `margin`). Those force reflow every frame and fail the 200ms INP budget. The
    Stylelint config already blocks new ones; find the ported ones.
@@ -44,4 +45,4 @@ query params, and reduced motion. If any fails, do not ship it.
 - [ ] Zero animations on layout properties
 - [ ] Every animation has a one-line written purpose
 - [ ] Reduced-motion tested under emulation; end states correct
-- [ ] Before/after captures attached; `npm run gate` green
+- [ ] Before/after captures attached; `pnpm run gate` green