Adopt pnpm as the package manager

Details

Replaces npm with pnpm so each git worktree gets its own node_modules
hardlinked from pnpm's global content-addressable store — instant, correct
per-branch installs instead of reinstalling or symlinking a shared tree.
- package.json: pin `packageManager: pnpm@10.34.4`; the `build` script now
  calls `pnpm run build:frontend` (was `npm run`); add
  `pnpm.onlyBuiltDependencies` (puppeteer, @parcel/watcher, protobufjs) since
  pnpm 10 blocks dependency build scripts by default.
- Lockfile: package-lock.json -> pnpm-lock.yaml (via `pnpm import`, exact
  resolved versions preserved).
- CI (6 workflows) + netlify.toml: add `pnpm/action-setup@v4` before
  setup-node, `cache: 'npm'` -> `'pnpm'`, `npm ci` ->
  `pnpm install --frozen-lockfile`, `npm run X` -> `pnpm run X`.
- Scripts (local.sh, verify_css_on_stop.sh, check_scss.sh) and docs
  (CLAUDE.md, READMEs, PREVIEW.md, DATABASE.md, agent prompts, cursor rules,
  tooling/test comments): npm -> pnpm.
Left as npm on purpose: the `npm i -g vercel@latest` CLI bootstrap in
deploy-vercel.yml (tool install; npm is always present on the runner) and the
cdn.jsdelivr.net/npm/... asset URLs (not package-manager commands).
Verified locally with pnpm 10.34.4: `pnpm install --frozen-lockfile` clean,
`pnpm run build` (tsc) ok, 16/16 core tests, `version:check` / `version:next`
ok. (`css:check` needs Node 22+ for sass's require() of the chokidar@5 ESM;
this sandbox runs Node 18, CI runs Node 24 — unrelated to the pnpm switch.)
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014c7dbCGbQxtWedKRSx4sJi

Files changed (31)