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-03-verification-net.md

@@ -1,8 +1,8 @@
 # Task 03 — Verification net
 
-**Agent**: `verification-engineer` · **Model**: Codex
-**Depends on**: 01 · **Parallel with**: 02, 04 · **Blocks**: 12–16
-**Worktree**: `.agents/scripts/worktree.sh start 03 verification-net`
+**Agent**: `verification-engineer` · **Model**: Codex **Depends on**: 01 ·
+**Parallel with**: 02, 04 · **Blocks**: 12–16 **Worktree**:
+`.agents/scripts/worktree.sh start 03 verification-net`
 
 ## Goal
 
@@ -14,11 +14,12 @@ This task is on the critical path. Do it early.
 
 ## Scope
 
-`.agents/snapshots/`, `scripts/audit-ui.mjs`, `.agents/scripts/visual-regression.mjs`.
+`.agents/snapshots/`, `scripts/audit-ui.mjs`,
+`.agents/scripts/visual-regression.mjs`.
 
 ## Steps
 
-1. `npm run serve` against the **current, unmigrated** site.
+1. `pnpm run serve` against the **current, unmigrated** site.
 2. Snapshot all ten routes:
    ```bash
    for r in "" full-guide summary models agents skills rules skills-review \
@@ -38,10 +39,12 @@ This task is on the critical path. Do it early.
 
 ## Done when
 
-- [ ] Ten committed snapshots, each non-empty and containing that page's real prose
+- [ ] Ten committed snapshots, each non-empty and containing that page's real
+      prose
 - [ ] `visual-regression.mjs` captures 10 routes × 4 widths
-- [ ] Extended `audit-ui.mjs` **fails** on today's `styles.css` (prove it catches the real bug), then the dead rule is removed by task 02
-- [ ] `npm run gate` green
+- [ ] Extended `audit-ui.mjs` **fails** on today's `styles.css` (prove it
+      catches the real bug), then the dead rule is removed by task 02
+- [ ] `pnpm run gate` green
 
 ## Do not