fix(ci): derive Windows standby store from workspace volume

.github/workflows/ci-master.yml

@@ -249,10 +249,9 @@ jobs:
       - name: Configure persistent pnpm store
         shell: pwsh
         # The store must share the ReFS workspace volume for the clone
-        # import method below; LOCALAPPDATA (C:) would cross volumes and
-        # break block clone. See 2026-08-30-windows-refs-store-block-clone-install.
+        # import method below. Runner workspaces can reside on different drives.
         run: |
-          $storeRoot = "F:\.pnpm-store"
+          $storeRoot = Join-Path ([IO.Path]::GetPathRoot($env:GITHUB_WORKSPACE)) '.pnpm-store'
           echo "PNPM_CONFIG_STORE_DIR=$storeRoot" >> $env:GITHUB_ENV
 
       - name: Install (immutable)

scripts/ci-workflow.spec.ts

@@ -258,7 +258,8 @@ describe('CI workflow', () => {
       isRecord(step) && step.name === 'Configure persistent pnpm store' && typeof step.run === 'string'
     ))
     expect(serialStore).toBeDefined()
-    expect(serialStore!.run).toContain('F:\\.pnpm-store')
+    expect(serialStore!.run).toContain('[IO.Path]::GetPathRoot($env:GITHUB_WORKSPACE)')
+    expect(serialStore!.run).toContain("'.pnpm-store'")
     const serialInstall = serialSteps.find((step): step is Record<string, unknown> & { run: string } => (
       isRecord(step) && step.name === 'Install (immutable)' && typeof step.run === 'string'
     ))