ci: finish the corepack sweep — pnpm/action-setup everywhere

Review follow-up on the symmetric-cache conversion: provisioning and
caching are separable concerns, so the remaining 8 corepack enable
sites (the three enterprise restore-only PR jobs, the required Windows
job, serial-macos/serial-windows, the cache-less ci.yml jobs, and
sandbox.yml) now provision pnpm via pnpm/action-setup@v4 too — only
the provisioning line moves; every deliberate cache shape (enterprise
restore-only pairing, serial-linux producer, Windows cache-less) is
byte-preserved. corepack is gone from CI entirely.

Agent Note retitled to the decision's real shape (provisioning policy,
with caching as per-job policy on top), EN+ZH, pair re-recorded;
composite-action and stop-at-cache-workflows alternatives recorded.

.github/workflows/sandbox.yml

@@ -53,13 +53,12 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      - uses: pnpm/action-setup@v4
+
       - uses: actions/setup-node@v6
         with:
           node-version: 24
 
-      - name: Enable corepack (pnpm)
-        run: corepack enable
-
       - name: Install (immutable)
         run: pnpm install --frozen-lockfile