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/skills/astro-component/SKILL.md

@@ -1,6 +1,9 @@
 ---
 name: astro-component
-description: Build one Astro component for the ai-for-dummies site from the project templates. Use when creating a primitive, block, or island, or when splitting existing markup into a component.
+description:
+  Build one Astro component for the ai-for-dummies site from the project
+  templates. Use when creating a primitive, block, or island, or when splitting
+  existing markup into a component.
 ---
 
 # Building an Astro component
@@ -38,6 +41,7 @@ interface Props {
 }
 const { number, title, summary, href } = Astro.props;
 ---
+
 <article class="card">
   <b>{number}</b>
   <h2>{title}</h2>
@@ -46,8 +50,16 @@ const { number, title, summary, href } = Astro.props;
 </article>
 
 <style>
-  .card { display: grid; gap: 10px; padding: 22px; background: var(--paper); }
-  b { color: var(--accent); font: var(--font-eyebrow); }
+  .card {
+    display: grid;
+    gap: 10px;
+    padding: 22px;
+    background: var(--paper);
+  }
+  b {
+    color: var(--accent);
+    font: var(--font-eyebrow);
+  }
 </style>
 ```
 
@@ -64,5 +76,6 @@ Rules that bite most often here:
 1. Render it at 560 / 800 / 1100 / 1600 px.
 2. Tab to it. Focus ring visible (`outline: 3px solid #a7483f`).
 3. Diff its rendered text against the markup it replaced.
-4. Run [`../../checklists/before-component.md`](../../checklists/before-component.md).
-5. `npm run verify` — green, with no assertion deleted.
+4. Run
+   [`../../checklists/before-component.md`](../../checklists/before-component.md).
+5. `pnpm run verify` — green, with no assertion deleted.