ci: Add uv installation to GitHub Actions workflow

Fix bootstrap test failures in CI by installing uv package manager.

The bootstrap script (scripts/bootstrap_skill.sh) requires uv to run,
which was causing 7 bootstrap E2E tests to fail in CI with:
  'uv: command not found'

Changes:
- Install uv via official installer (works on Ubuntu & macOS)
- Add uv to PATH for subsequent steps

This resolves the failing tests from PR #251 merge.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/tests.yml

@@ -29,6 +29,11 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
+    - name: Install uv
+      run: |
+        curl -LsSf https://astral.sh/uv/install.sh | sh
+        echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
     - name: Cache pip packages
       uses: actions/cache@v3
       with: