ci: add Blacksmith sticky-disk cache for install step

useblacksmith/cache@v1 before install — caches the package manager's
download cache on Blacksmith's persistent NVMe disk. ~2-3× faster
installs on warm reruns.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vvillait88

.github/workflows/ci.yml

@@ -15,6 +15,13 @@ jobs:
     steps:
       - uses: useblacksmith/checkout@v1
       - uses: astral-sh/setup-uv@v7
+      - uses: useblacksmith/cache@v1
+        with:
+          path: |
+            ~/.cache/uv
+            ~/.local/share/uv
+          key: ${{ runner.os }}-uv-${{ hashFiles('uv.lock') }}
+          restore-keys: ${{ runner.os }}-uv-
       - run: uv sync --frozen --all-extras
       - run: uv run ruff check .
       - run: uv run ruff format --check .