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/scripts/check-tokens.mjs

@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 // Fails when a raw colour, px font-size, or ad-hoc breakpoint appears outside
 // the token layer. A rule nobody checks is a suggestion — wire this into
-// `npm run verify`.
+// `pnpm run verify`.
 //
 // Usage: node .agents/scripts/check-tokens.mjs [srcDir]
 
@@ -29,26 +29,30 @@ for (const path of targets) {
   if (!['.astro', '.css'].includes(extname(path))) continue;
   if (TOKEN_FILES.some((allowed) => path.endsWith(allowed))) continue;
 
-  readFileSync(path, 'utf8').split('\n').forEach((line, index) => {
-    const at = `${path}:${index + 1}`;
+  readFileSync(path, 'utf8')
+    .split('\n')
+    .forEach((line, index) => {
+      const at = `${path}:${index + 1}`;
 
-    // Raw hex — the drifted-palette failure mode this whole layer exists to stop.
-    const hex = line.match(/#[0-9a-fA-F]{3,8}\b/g);
-    if (hex) findings.push(`${at}: raw hex ${hex.join(', ')} — use a token from tokens.css`);
+      // Raw hex — the drifted-palette failure mode this whole layer exists to stop.
+      const hex = line.match(/#[0-9a-fA-F]{3,8}\b/g);
+      if (hex) findings.push(`${at}: raw hex ${hex.join(', ')} — use a token from tokens.css`);
 
-    // rgb()/hsl() literals are the same problem wearing a different hat.
-    if (/\b(rgba?|hsla?)\(\s*\d/.test(line))
-      findings.push(`${at}: raw colour function — use a token`);
+      // rgb()/hsl() literals are the same problem wearing a different hat.
+      if (/\b(rgba?|hsla?)\(\s*\d/.test(line))
+        findings.push(`${at}: raw colour function — use a token`);
 
-    // Hard-coded font sizes bypass the type scale.
-    const fontSize = line.match(/font-size:\s*\d+(\.\d+)?px/);
-    if (fontSize) findings.push(`${at}: hard-coded ${fontSize[0]} — use var(--step-*)`);
+      // Hard-coded font sizes bypass the type scale.
+      const fontSize = line.match(/font-size:\s*\d+(\.\d+)?px/);
+      if (fontSize) findings.push(`${at}: hard-coded ${fontSize[0]} — use var(--step-*)`);
 
-    // Ad-hoc breakpoints are how sixteen of them accumulated last time.
-    const media = line.match(/@media[^{]*?\(\s*(?:max|min)-width:\s*(\d+px)/);
-    if (media && !ALLOWED_BREAKPOINTS.includes(media[1]))
-      findings.push(`${at}: breakpoint ${media[1]} is not a named one (${ALLOWED_BREAKPOINTS.join(', ')})`);
-  });
+      // Ad-hoc breakpoints are how sixteen of them accumulated last time.
+      const media = line.match(/@media[^{]*?\(\s*(?:max|min)-width:\s*(\d+px)/);
+      if (media && !ALLOWED_BREAKPOINTS.includes(media[1]))
+        findings.push(
+          `${at}: breakpoint ${media[1]} is not a named one (${ALLOWED_BREAKPOINTS.join(', ')})`,
+        );
+    });
 }
 
 if (findings.length) {