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/theming.md

@@ -15,7 +15,7 @@ the token layer is missing something (add it there, not inline).
 ```css
 /* forbidden */
 color: #172f42;
-color: rgba(23,47,66,.6);
+color: rgba(23, 47, 66, 0.6);
 
 /* required */
 color: var(--ink);
@@ -23,7 +23,7 @@ color: var(--muted);
 ```
 
 No raw hex outside `tokens.css`. `.agents/scripts/check-tokens.mjs` enforces it;
-wire it into `npm run verify`.
+wire it into `pnpm run verify`.
 
 ## Semantic names, not literal ones
 
@@ -31,20 +31,20 @@ wire it into `npm run verify`.
 `--deep` are the existing vocabulary. Keep it — it is already semantic and the
 team reads it fluently. Do not rename to `--color-neutral-900`.
 
-If a genuine second surface is needed, extend semantically
-(`--surface-lab`, `--ink-inverse`), never numerically.
+If a genuine second surface is needed, extend semantically (`--surface-lab`,
+`--ink-inverse`), never numerically.
 
 ## Type scale
 
 Replace the 14 ad-hoc `clamp()` triples with named steps:
 
 ```css
---step-display: clamp(56px, 9vw, 126px);   /* h1 */
---step-6: clamp(36px, 5vw, 65px);          /* section h2 */
---step-5: clamp(24px, 3vw, 38px);          /* sub-head */
---step-4: clamp(22px, 3vw, 36px);          /* pull-quote */
---step-1: 15px;                            /* body */
---step-0: 11px;                            /* eyebrow / label */
+--step-display: clamp(56px, 9vw, 126px); /* h1 */
+--step-6: clamp(36px, 5vw, 65px); /* section h2 */
+--step-5: clamp(24px, 3vw, 38px); /* sub-head */
+--step-4: clamp(22px, 3vw, 36px); /* pull-quote */
+--step-1: 15px; /* body */
+--step-0: 11px; /* eyebrow / label */
 ```
 
 The eyebrow treatment (`10–11px` monospace, `letter-spacing:.08–.1em`,
@@ -56,8 +56,11 @@ repetitions.
 Five named widths replace the current sixteen:
 
 ```css
---bp-sm: 560px;  --bp-md: 800px;  --bp-lg: 1100px;
---bp-xl: 1600px; --bp-2xl: 2200px;
+--bp-sm: 560px;
+--bp-md: 800px;
+--bp-lg: 1100px;
+--bp-xl: 1600px;
+--bp-2xl: 2200px;
 ```
 
 When collapsing a component's old breakpoint onto a named one, screenshot at the
@@ -74,5 +77,5 @@ When collapsing a component's old breakpoint onto a named one, screenshot at the
 ## Fonts
 
 Do not add a webfont without an explicit decision recorded in the task. The
-intended Manrope/DM Mono has never rendered; introducing it is a visual redesign,
-not a refactor. Default: match what renders today.
+intended Manrope/DM Mono has never rendered; introducing it is a visual
+redesign, not a refactor. Default: match what renders today.