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/visual-regression/SKILL.md

@@ -1,6 +1,8 @@
 ---
 name: visual-regression
-description: Prove a refactor did not change how the site looks. Use before and after any page migration, token consolidation, or breakpoint change on ai-for-dummies.
+description:
+  Prove a refactor did not change how the site looks. Use before and after any
+  page migration, token consolidation, or breakpoint change on ai-for-dummies.
 ---
 
 # Visual regression
@@ -31,8 +33,8 @@ with sync_playwright() as p:
     browser.close()
 ```
 
-Run once against the vanilla site (`npm run serve`), once against
-`npm run preview`. Keep both sets.
+Run once against the vanilla site (`pnpm run serve`), once against
+`pnpm run preview`. Keep both sets.
 
 ## Compare
 
@@ -43,8 +45,8 @@ for f in before/*.png; do
 done
 ```
 
-Pixel-exact is not the bar — antialiasing differs. Judge by eye where the
-metric is non-trivial, and attach the pair to the task report.
+Pixel-exact is not the bar — antialiasing differs. Judge by eye where the metric
+is non-trivial, and attach the pair to the task report.
 
 ## The three widths that catch the most
 
@@ -61,9 +63,9 @@ Also screenshot at the **old** breakpoint values you removed (520, 530, 600,
 
 Expect and accept: sub-pixel text shifts, antialiasing.
 
-Investigate: anything that moves by more than ~2px, any colour change (that is
-a token bug), any element that appears or disappears (that is content loss —
-stop and check the snapshot diff).
+Investigate: anything that moves by more than ~2px, any colour change (that is a
+token bug), any element that appears or disappears (that is content loss — stop
+and check the snapshot diff).
 
 ## Reduced motion