Switch package manager from Yarn to pnpm (#214)

* Switch package manager from Yarn to pnpm

Replace Yarn 3 (Berry) with pnpm 11.12.0 (latest stable):

- Pin packageManager to [email protected].0; add pnpm-lock.yaml
- Remove yarn.lock, .yarnrc.yml, and the bundled .yarn/ release
- Update CI (ci-test, deploy) to use pnpm/action-setup + setup-node
  pnpm cache, with `pnpm install --frozen-lockfile` and pnpm scripts
- Switch Husky pre-commit hook to `pnpm check`
- Add pnpm-workspace.yaml allowlisting esbuild's build script, which
  pnpm 11 otherwise blocks with a hard error on install
- Update CLAUDE.md guidance and drop yarn-specific .gitignore entries

Verified: pnpm install --frozen-lockfile, pnpm check, and pnpm build
all pass locally.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* CI: read Node version from .tool-versions for pnpm 11 compat

pnpm 11.12.0 requires Node >=22.13, but the workflows pinned
node-version: '20' (20.20.2), failing install. Point setup-node at
.tool-versions (22.14.0) so CI and local dev share one source of truth.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>

.gitignore

@@ -2,8 +2,6 @@
 logs
 *.log
 npm-debug.log*
-yarn-debug.log*
-yarn-error.log*
 pnpm-debug.log*
 lerna-debug.log*
 
@@ -22,12 +20,3 @@ dist-ssr
 *.njsproj
 *.sln
 *.sw?
-
-# Yarn 2+
-.pnp.*
-.yarn/*
-!.yarn/patches
-!.yarn/plugins
-!.yarn/releases
-!.yarn/sdks
-!.yarn/versions