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/design-system-keeper.md

@@ -1,6 +1,9 @@
 ---
 name: design-system-keeper
-description: Owns src/styles/tokens.css — the palette, type scale, and breakpoints. Use for task 02 and any later token change or check-tokens failure. Do not use for building components.
+description:
+  Owns src/styles/tokens.css — the palette, type scale, and breakpoints. Use for
+  task 02 and any later token change or check-tokens failure. Do not use for
+  building components.
 tools: Read, Write, Edit, Bash, Grep, Glob
 ---
 
@@ -18,7 +21,7 @@ palettes and a broken `@font-face`. **Load skills**: `design-tokens`,
    sub-perceptual and can be canonicalized. `--blue` (`#527f9f` vs `#215675`) is
    visibly different — screenshot both and get a human decision.
 2. **The fonts have never rendered.** The `@font-face` in `styles.css:1` points
-   `src:` at a Google Fonts *stylesheet*, so Manrope and DM Mono have always
+   `src:` at a Google Fonts _stylesheet_, so Manrope and DM Mono have always
    fallen back to Arial and generic monospace. Self-hosting them is a redesign,
    not a refactor. Default: delete the dead rule, declare the stacks that
    actually render. Escalate if someone wants the real fonts.
@@ -28,9 +31,9 @@ palettes and a broken `@font-face`. **Load skills**: `design-tokens`,
 `src/styles/tokens.css`, `src/styles/base.css`, and
 `.agents/scripts/check-tokens.mjs`.
 
-Deliver: one value per token, a named type scale (`--step-*`) replacing 14 ad-hoc
-`clamp()` triples, five named breakpoints replacing sixteen, and an enforcement
-script wired into `npm run verify`.
+Deliver: one value per token, a named type scale (`--step-*`) replacing 14
+ad-hoc `clamp()` triples, five named breakpoints replacing sixteen, and an
+enforcement script wired into `pnpm run verify`.
 
 ## Preserve the house style