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/rules/git-worktrees.md

@@ -8,7 +8,7 @@ This project teaches worktrees. It should use them properly.
 # from the main checkout
 git worktree add ../af-task-07 -b refactor/task-07-route-cards
 cd ../af-task-07
-npm ci
+pnpm install --frozen-lockfile
 ```
 
 Naming: directory `../af-task-NN`, branch `refactor/task-NN-<slug>`. Both
@@ -18,8 +18,8 @@ derived from the task file so the mapping is never ambiguous.
 
 The migration runs many agents in parallel over the same small set of files
 (`tokens.css`, `verify.mjs`, `astro.config.mjs` are contended). Worktrees give
-each agent its own working directory over one object store — cheap, and no
-agent can see another's half-finished state.
+each agent its own working directory over one object store — cheap, and no agent
+can see another's half-finished state.
 
 The failure mode without them: two agents both "fix" `verify.mjs`, and the
 second overwrites the first's assertions.
@@ -29,12 +29,12 @@ second overwrites the first's assertions.
 These are touched by many tasks. Whoever owns them per the plan is the **only**
 writer; everyone else opens an issue in their task report instead of editing:
 
-| File | Owner |
-| --- | --- |
-| `src/styles/tokens.css` | `design-system-keeper` |
-| `scripts/verify.mjs` | `verification-engineer` |
-| `astro.config.mjs`, `package.json` | `astro-architect` |
-| `src/content/config.ts` | `content-i18n-migrator` |
+| File                               | Owner                   |
+| ---------------------------------- | ----------------------- |
+| `src/styles/tokens.css`            | `design-system-keeper`  |
+| `scripts/verify.mjs`               | `verification-engineer` |
+| `astro.config.mjs`, `package.json` | `astro-architect`       |
+| `src/content/config.ts`            | `content-i18n-migrator` |
 
 ## Before you start
 
@@ -44,7 +44,7 @@ writer; everyone else opens an issue in their task report instead of editing:
 
 ## Before you finish
 
-1. `npm run verify` green — without deleting assertions.
+1. `pnpm run verify` green — without deleting assertions.
 2. The relevant checklist in [`../checklists/`](../checklists/) complete.
 3. `git rebase origin/main` again, resolve conflicts in your worktree.
 4. Task report: what changed, what you verified, **what you did not do**.
@@ -64,6 +64,8 @@ should be short.
 
 - Never work directly on `main`.
 - Never force-push a shared branch. The `pages` branch is the sole exception,
-  and only if CI owns it (see [`../context/publishing.md`](../context/publishing.md)).
+  and only if CI owns it (see
+  [`../context/publishing.md`](../context/publishing.md)).
 - Never `git add -A` from the repository root. This repo has untracked local
-  scratch (`.serena/`, `scripts/inspect.py`) that must not be swept into a commit.
+  scratch (`.serena/`, `scripts/inspect.py`) that must not be swept into a
+  commit.