Use shared render action as PR merge gate

[Copilot]

This updates PR render checks so merge acceptance is driven by the shared render action path, aligned with repository rendering behavior. The PR workflow now invokes the composite render action directly instead of maintaining a separate inline render pipeline.

• Workflow gate alignment

  • .github/workflows/render.yml now uses ./.github/actions/render for the PR render job.
  • This makes the render check the canonical merge criterion via the reusable action.

• Refactor of PR render workflow

  • Removed duplicated inline Nix install/cache/export and compile steps from the workflow.
  • Kept base_ref setup in the workflow so protocol conditional build logic remains available to the composite action.

• Resulting execution path

  • PR renderability checks and shared render implementation now run through one maintained action surface.

- name: Set base ref
  run: |
    git show --format=%H --no-notes --no-patch "HEAD~1" -- |tee base_ref
    if ! ( grep -E '^[0-9a-f]{40}$' base_ref ); then exit 1; fi

- name: Compile ZIPs and Zcash Protocol Specification
  uses: ./.github/actions/render