ci: install pnpm where format-check runs

`uv run poe format-check` now invokes `pnpm exec prettier` for the
markdown step, but the CI test job, the Copilot setup-steps job, and
the devcontainer onCreate script only had the Python toolchain
installed. Add pnpm + Node setup (CI) and corepack enable +
`pnpm install --frozen-lockfile` (devcontainer) so prettier is
available wherever format-check runs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dougborg

.devcontainer/oncreate.sh

@@ -16,6 +16,12 @@ uv --version
 echo "📚 Syncing project dependencies (will be cached)..."
 uv sync --all-extras
 
+# Install Node dependencies (needed for prettier — used by markdown
+# format hook and `uv run poe format-markdown`).
+echo "🟢 Enabling corepack and installing Node dependencies..."
+corepack enable
+pnpm install --frozen-lockfile
+
 # Install pre-commit hooks
 echo "🪝 Installing pre-commit hooks..."
 uv run pre-commit install

.github/workflows/ci.yml

@@ -62,9 +62,22 @@ jobs:
           cache-dependency-glob: "uv.lock"
           python-version: ${{ matrix.python-version }}
 
+      - name: Install pnpm
+        if: needs.changes.outputs.code == 'true'
+        uses: pnpm/action-setup@v6
+
+      - name: Install Node.js
+        if: needs.changes.outputs.code == 'true'
+        uses: actions/setup-node@v6
+        with:
+          node-version: "20"
+          cache: "pnpm"
+
       - name: Install dependencies
         if: needs.changes.outputs.code == 'true'
-        run: uv sync --all-extras
+        run: |
+          uv sync --all-extras
+          pnpm install --frozen-lockfile
 
       - name: Check code formatting
         if: needs.changes.outputs.code == 'true'

.github/workflows/copilot-setup-steps.yml

@@ -39,11 +39,22 @@ jobs:
           cache-dependency-glob: "uv.lock"
           python-version: "3.14"
 
+      - name: Install pnpm
+        uses: pnpm/action-setup@v6
+
+      - name: Install Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: "20"
+          cache: "pnpm"
+
       - name: Install dependencies
         run: |
           echo "🔧 Installing project dependencies with uv..."
           echo "⏱️  This should only take a few seconds..."
           uv sync --all-extras
+          echo "📦 Installing Node deps with pnpm (needed for prettier)..."
+          pnpm install --frozen-lockfile
           echo "✅ Dependencies installed successfully"
 
       - name: Install pre-commit hooks