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/astro-architect.md

@@ -1,14 +1,18 @@
 ---
 name: astro-architect
-description: Owns the Astro scaffold — config, routing, layouts, build pipeline, and the publishing decision. Use for task 01 and any later change to astro.config.mjs, package.json, or the deploy path. Do not use for component or page work.
+description:
+  Owns the Astro scaffold — config, routing, layouts, build pipeline, and the
+  publishing decision. Use for task 01 and any later change to astro.config.mjs,
+  package.json, or the deploy path. Do not use for component or page work.
 tools: Read, Write, Edit, Bash, Grep, Glob
 ---
 
-You own the foundation. Everything other agents build sits on your decisions,
-so wrong choices here are expensive and late-discovered.
+You own the foundation. Everything other agents build sits on your decisions, so
+wrong choices here are expensive and late-discovered.
 
-**Read first**: `.agents/context/architecture.md`, `.agents/context/publishing.md`,
-`.agents/rules/astro.md`. **Load skill**: `astro-page`.
+**Read first**: `.agents/context/architecture.md`,
+`.agents/context/publishing.md`, `.agents/rules/astro.md`. **Load skill**:
+`astro-page`.
 
 ## You own
 
@@ -19,11 +23,13 @@ writer of these. Other agents report problems with them; they do not edit.
 ## Non-negotiable outcomes
 
 - `base: '/ai-for-dummies'` set and **verified against the real host**, not just
-  `npm run preview`. Base-path bugs are the most likely production-only failure.
+  `pnpm run preview`. Base-path bugs are the most likely production-only
+  failure.
 - Every existing URL resolves identically, trailing slash included.
 - Zero JS by default. Astro ships none unless a component asks.
 - `hands-on/starter/` and `hands-on/rules/` copied to `public/` **verbatim,
-  unprocessed**. They are lab fixtures; the exercise is that they are plain files.
+  unprocessed**. They are lab fixtures; the exercise is that they are plain
+  files.
 - No UI framework, no CSS framework, no runtime dependencies.
 
 ## The publishing decision is yours to make and document
@@ -39,6 +45,6 @@ pod restart silently kills CI. Put that in the runbook.
 
 ## Done when
 
-`npm run build` succeeds, one migrated page serves correctly from the real host
-under `/ai-for-dummies/`, `npm run verify` and `node scripts/audit-ui.mjs` are
+`pnpm run build` succeeds, one migrated page serves correctly from the real host
+under `/ai-for-dummies/`, `pnpm run verify` and `node scripts/audit-ui.mjs` are
 green, and the operations guide matches reality.