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]>

.github/workflows/ci-test.yml

@@ -14,13 +14,14 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version-file: '.tool-versions'
+          cache: 'pnpm'
       - name: Install Dependencies
-        run: |
-          npm install -g yarn
-          yarn install
+        run: pnpm install --frozen-lockfile
       - name: Lint solution
-        run: yarn check
+        run: pnpm check

.github/workflows/deploy.yml

@@ -13,16 +13,17 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version-file: '.tool-versions'
+          cache: 'pnpm'
       - name: Install Dependencies
-        run: |
-          npm install -g yarn
-          yarn install
+        run: pnpm install --frozen-lockfile
       - name: Build solution
-        run: yarn build
+        run: pnpm build
       - name: Archive artifacts
         uses: actions/[email protected]
         with: