ci: pin pnpm store path across runner fleets (#6414)

* ci: pin pnpm store path across runner fleets

Nexu ARC resolved the store under _work while hosted/seed used the home
store, so actions/cache version hashes diverged and every Nexu job missed.
Force $HOME/.pnpm-store and add restore-keys prefix fallback.

* test(e2e): lock stable pnpm store path and restore-keys

Add regression assertions for the non-persistent home store pin and
OS-prefix restore-keys so the cross-fleet cache contract cannot regress
silently.

Generated-By: looper 0.11.8 (runner=fixer, agent=grok-build)

* ci: seed Linux pnpm cache from cache-maintenance

Pinning the store path alone cannot hit until a main job saves under the
new actions/cache version. Add a single ubuntu-24.04 seed job; the cache
is repo-scoped so every Nexu size reuses it.

.github/actions/setup-workspace/action.yml

@@ -47,7 +47,17 @@ runs:
           echo "enabled=true" >> "$GITHUB_OUTPUT"
           echo "path=$store_dir" >> "$GITHUB_OUTPUT"
         else
+          # Force a stable home store path so actions/cache version hashes match
+          # across fleets (hosted vs Nexu ARC). ARC may copy instead of hardlink;
+          # that is acceptable for cache hit stability.
+          store_dir="$HOME/.pnpm-store"
+          mkdir -p "$store_dir"
+          {
+            echo "NPM_CONFIG_STORE_DIR=$store_dir"
+            echo "npm_config_store_dir=$store_dir"
+          } >> "$GITHUB_ENV"
           echo "enabled=false" >> "$GITHUB_OUTPUT"
+          echo "path=$store_dir" >> "$GITHUB_OUTPUT"
         fi
 
     - name: Setup pnpm
@@ -73,6 +83,8 @@ runs:
       with:
         path: ${{ steps.pnpm-store.outputs.path }}
         key: pnpm-store-${{ runner.os }}-${{ hashFiles('pnpm-lock.yaml') }}
+        restore-keys: |
+          pnpm-store-${{ runner.os }}-
 
     - name: Install dependencies
       shell: bash