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-page/SKILL.md

@@ -1,6 +1,9 @@
 ---
 name: astro-page
-description: Migrate one hand-written HTML page of the ai-for-dummies site to an Astro route without changing its URL, content, or JS budget. Use for any page-level migration task.
+description:
+  Migrate one hand-written HTML page of the ai-for-dummies site to an Astro
+  route without changing its URL, content, or JS budget. Use for any page-level
+  migration task.
 ---
 
 # Migrating a page to Astro
@@ -10,7 +13,7 @@ description: Migrate one hand-written HTML page of the ai-for-dummies site to an
 The snapshot is the only objective evidence that no content was lost.
 
 ```bash
-npm run serve &                                  # vanilla site on :4173
+pnpm run serve &                                  # vanilla site on :4173
 node .agents/scripts/snapshot-route.mjs http://localhost:4173/models/ \
   > .agents/snapshots/models.txt
 ```
@@ -30,7 +33,8 @@ Then, in order:
 2. Move copy into `src/content/`. Both `en` and `pt`, copied literally — never
    retyped.
 3. Move page CSS into component `<style>` blocks. Do **not** port
-   `responsive.css` wholesale; take what this page needs and prove the rest dead.
+   `responsive.css` wholesale; take what this page needs and prove the rest
+   dead.
 4. Keep every `data-*` hook. `verify.mjs` asserts many of them by name
    (`data-phase`, `data-tree`, `data-route`, `data-model-provider`, …).
 5. Keep every ARIA attribute and the `<title>` / `<meta name="description">`.
@@ -49,11 +53,11 @@ documented in the page footer and shared externally.
 ## Prove it
 
 ```bash
-npm run build
+pnpm run build
 node .agents/scripts/snapshot-route.mjs dist/models/index.html > /tmp/after.txt
 diff .agents/snapshots/models.txt /tmp/after.txt      # empty, or justify every line
 node scripts/audit-ui.mjs
-npm run verify
+pnpm run verify
 ```
 
 Then screenshots at the same four widths, and