ci: move release smoke-test linux leg to blacksmith

Step 4 of the Blacksmith rollout from CLI-1497. Replaces ubuntu-latest
with blacksmith-8vcpu-ubuntu-2404 in the smoke-test matrix. The Linux
leg pulls 6 images x 2 platforms via docker; Blacksmith runners ship a
local registry mirror that makes those pulls near-instant, so we rely
on it instead of any explicit pre-pull/cache machinery.

The QEMU setup stays -- arm64 docker subtests still execute via
emulation. A future PR can split the leg into native amd64 + arm64
Blacksmith runners (per the parent plan's PR7).

The macOS and Windows entries are unchanged; Blacksmith is Linux-only.

Refs CLI-1501.

Author: claude

.github/workflows/release-shared.yml

@@ -101,7 +101,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        runner: [ubuntu-latest, macos-latest, macos-15-intel, windows-latest]
+        runner: [blacksmith-8vcpu-ubuntu-2404, macos-latest, macos-15-intel, windows-latest]
     runs-on: ${{ matrix.runner }}
     env:
       NPM_TAG: ${{ inputs.npm_tag }}