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>

plans/astro-refactor/task-01-scaffold.md

@@ -1,7 +1,8 @@
 # Task 01 — Astro scaffold, gates, and the publishing decision
 
 **Agent**: `astro-architect` · **Model**: Codex · **Depends on**: nothing
-**Blocks**: everything · **Worktree**: `.agents/scripts/worktree.sh start 01 scaffold`
+**Blocks**: everything · **Worktree**:
+`.agents/scripts/worktree.sh start 01 scaffold`
 
 ## Goal
 
@@ -10,16 +11,16 @@ host under `/ai-for-dummies/`, and has all three gate tiers live.
 
 ## Scope
 
-`astro.config.mjs`, `package.json`, `tsconfig.json`, `src/layouts/BaseLayout.astro`,
-`.husky/`, `.lintstagedrc.json`, lint configs, `.gitea/workflows/verify.yml`,
-`docs/operations-guide.md`.
+`astro.config.mjs`, `package.json`, `tsconfig.json`,
+`src/layouts/BaseLayout.astro`, `.husky/`, `.lintstagedrc.json`, lint configs,
+`.gitea/workflows/verify.yml`, `docs/operations-guide.md`.
 
 Copy the configs from `.agents/templates/config/` — they are written for this
 project (correct ignores for `hands-on/`, `submitted-skills/`, `vote-service/`).
 
 ## Steps
 
-1. `npm create astro@latest` into the worktree — minimal template, TypeScript
+1. `pnpm create astro@latest` into the worktree — minimal template, TypeScript
    strict, **no UI framework, no CSS framework**.
 2. Set `base: '/ai-for-dummies'`. Every internal link goes through
    `import.meta.env.BASE_URL` from here on.
@@ -28,22 +29,25 @@ project (correct ignores for `hands-on/`, `submitted-skills/`, `vote-service/`).
 4. Merge `.agents/templates/config/package.scripts.json` into `package.json`.
    **`"prepare": "husky"` is what makes hooks exist** — without it every hook is
    inert.
-5. `npm install husky lint-staged prettier eslint stylelint …`, then `npx husky init`.
+5. `pnpm install husky lint-staged prettier eslint stylelint …`, then
+   `pnpm exec husky init`.
 6. Copy the three hooks and the lint configs into place. Verify with
    `.agents/scripts/verify-hooks.sh`.
 7. Migrate **one** page (`summary/` — smallest, zero JS) as a smoke test.
-8. Copy `hands-on/` into `public/` (task 17 does this properly; a stub is fine here).
-9. **Make the publishing decision** per `.agents/context/publishing.md`. Recommended:
-   Gitea Actions builds `dist/` and pushes `pages`. Copy
+8. Copy `hands-on/` into `public/` (task 17 does this properly; a stub is fine
+   here).
+9. **Make the publishing decision** per `.agents/context/publishing.md`.
+   Recommended: Gitea Actions builds `dist/` and pushes `pages`. Copy
    `.agents/templates/config/gitea-ci.yaml` to `.gitea/workflows/verify.yml`.
 10. Rewrite the publishing section of `docs/operations-guide.md` to match.
 
 ## Done when
 
-- [ ] `npm run build` succeeds; `npm run gate` passes
+- [ ] `pnpm run build` succeeds; `pnpm run gate` passes
 - [ ] `.agents/scripts/verify-hooks.sh` reports hooks live
 - [ ] A bad commit message is rejected; a raw hex in a `.astro` file is rejected
-- [ ] `/ai-for-dummies/summary/` serves correctly **from the real host**, not just preview
+- [ ] `/ai-for-dummies/summary/` serves correctly **from the real host**, not
+      just preview
 - [ ] `docs/operations-guide.md` describes the actual publishing path
 
 ## Do not
@@ -54,6 +58,6 @@ project (correct ignores for `hands-on/`, `submitted-skills/`, `vote-service/`).
 
 ## Watch for
 
-The base path is the #1 production-only failure in this migration. `npm run
-preview` will lie to you. Deploy the smoke-test page and curl it with a
-`?v=<sha>` cache-buster.
+The base path is the #1 production-only failure in this migration.
+`pnpm run preview` will lie to you. Deploy the smoke-test page and curl it with
+a `?v=<sha>` cache-buster.