fix(ci): disable package-manager-cache on skip-install setup-node

actions/setup-node@v5 defaults package-manager-cache:true, so the agent-browser
pack fixture still registered a pnpm store path and failed post-job with Path
Validation Error after a successful pack/upload. Explicitly set
package-manager-cache:false when skip-install is true.

.github/actions/setup-node-pnpm/action.yml

@@ -39,6 +39,13 @@ runs:
     # "Path Validation Error: Path(s) specified in the action for caching do(es)
     # not exist" and marks an otherwise-successful pack job failed. Only enable
     # the pnpm store cache when this composite will actually install.
+    #
+    # FNXC:CI 2026-08-03-06:26:
+    # Omitting `cache: pnpm` is NOT enough on actions/setup-node@v5+: the default
+    # `package-manager-cache: true` still auto-detects the workspace lockfile, registers
+    # the pnpm store path for post-job save, and red-X's skip-install jobs after a green
+    # pack (agent-browser-install pack-fixture on PR #3307). Explicitly disable it on the
+    # no-install path; keep the install path on the explicit `cache: pnpm` contract.
     - name: Setup Node.js
       if: ${{ inputs.skip-install != 'true' }}
       uses: actions/setup-node@v5
@@ -52,6 +59,7 @@ runs:
       uses: actions/setup-node@v5
       with:
         node-version: ${{ inputs.node-version }}
+        package-manager-cache: false
         registry-url: ${{ inputs.registry-url }}
 
     - name: Install dependencies

packages/cli/src/__tests__/ci-workflow.test.ts

@@ -264,6 +264,11 @@ describe("Merge gate (.github/workflows/pr-checks.yml)", () => {
   skip-install pack jobs never create a pnpm store; setup-node must not enable
   cache: pnpm on that path or post-job cache save fails the whole job after a
   successful pack (agent-browser-install pack-fixture).
+
+  FNXC:CI 2026-08-03-06:26:
+  setup-node@v5+ also defaults package-manager-cache:true, which reintroduces the
+  same Path Validation Error even when cache is omitted — pin it false on the
+  no-install path (PR #3307 agent-browser pack fixture post-step).
   */
   it("disables pnpm store cache when skip-install is true", () => {
     const setupSteps = (compositeAction.runs?.steps ?? []).filter(
@@ -276,6 +281,7 @@ describe("Merge gate (.github/workflows/pr-checks.yml)", () => {
     expect(withoutCache?.if).toContain("skip-install");
     expect(withoutCache?.if).toContain("==");
     expect(withoutCache?.with?.cache).toBeUndefined();
+    expect(withoutCache?.with?.["package-manager-cache"]).toBe(false);
   });
 
   it("keeps lint as install + lint only, without Bun/setup build coupling", () => {