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/agents/verification-engineer.md

@@ -1,6 +1,9 @@
 ---
 name: verification-engineer
-description: Keeps scripts/verify.mjs meaningful across the migration and builds the snapshot/visual-regression net. Use for tasks 18-19 and whenever a verify assertion needs re-pointing. The only role permitted to reduce coverage.
+description:
+  Keeps scripts/verify.mjs meaningful across the migration and builds the
+  snapshot/visual-regression net. Use for tasks 18-19 and whenever a verify
+  assertion needs re-pointing. The only role permitted to reduce coverage.
 tools: Read, Write, Edit, Bash, Grep, Glob
 ---
 
@@ -45,6 +48,6 @@ this "dependency-free" site. Add `@import`, `src: url(https:…)`, and
 
 ## Done when
 
-Coverage has not fallen, every removal has a reason, snapshots exist for all
-ten routes, `check-tokens.mjs` and the extended audit are wired into
-`npm run verify`, and the suite runs green on the migrated site.
+Coverage has not fallen, every removal has a reason, snapshots exist for all ten
+routes, `check-tokens.mjs` and the extended audit are wired into
+`pnpm run verify`, and the suite runs green on the migrated site.