ci: adopt pnpm/action-setup for symmetric pnpm store caching

Convert the hand-rolled corepack + store-path + actions/cache pnpm setups
to pnpm/action-setup@v4 + setup-node cache: pnpm (the landlock-run.yml
pattern) in e2e.yml, docs-pages.yml, pi-ai-provider-e2e.yml,
build-exe-for-python-sdk.yml, and ci.yml's node-compat and benchmark
jobs. The enterprise restore-only PR jobs and the Windows job keep their
documented asymmetries; serial-linux keeps its hand-rolled store cache
because it produces the exact key those restore-only jobs consume, and
only swaps corepack for the action.

Implements and moves the CI-caching Agent Note from PR #667 to
implemented/, recording the serial-linux producer exception and the
exact-key (no restore-keys prefix) behavior of setup-node's cache.

.github/workflows/docs-pages.yml

@@ -33,23 +33,12 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      - uses: pnpm/action-setup@v4
+
       - uses: actions/setup-node@v6
         with:
           node-version: ${{ env.PRIMARY_NODE_VERSION }}
-
-      - name: Enable corepack (pnpm)
-        run: corepack enable
-
-      - name: Resolve pnpm store path
-        id: pnpm-store
-        run: echo "path=$(pnpm store path --silent)" >> "$GITHUB_OUTPUT"
-
-      - uses: actions/cache@v4
-        with:
-          path: ${{ steps.pnpm-store.outputs.path }}
-          key: ${{ runner.os }}-node-${{ env.PRIMARY_NODE_VERSION }}-pnpm-${{ hashFiles('pnpm-lock.yaml') }}
-          restore-keys: |
-            ${{ runner.os }}-node-${{ env.PRIMARY_NODE_VERSION }}-pnpm-
+          cache: pnpm
 
       - name: Install (immutable)
         run: pnpm install --frozen-lockfile