chore: cleanup unused files and fix GitHub Actions warnings

- Remove unused packages/leann-backend-diskann/CMakeLists.txt
  (DiskANN uses cmake.source-dir=third_party/DiskANN instead)
- Replace macos-latest with macos-14 to avoid migration warnings
  (macos-latest will migrate to macOS 15 on August 4, 2025)
- Keep packages/leann-backend-hnsw/CMakeLists.txt (needed for Faiss config)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.github/workflows/build-reusable.yml

@@ -54,15 +54,15 @@ jobs:
             python: '3.12'
           - os: ubuntu-22.04
             python: '3.13'
-          - os: macos-latest
+          - os: macos-14
             python: '3.9'
-          - os: macos-latest
+          - os: macos-14
             python: '3.10'
-          - os: macos-latest
+          - os: macos-14
             python: '3.11'
-          - os: macos-latest
+          - os: macos-14
             python: '3.12'
-          - os: macos-latest
+          - os: macos-14
             python: '3.13'
           - os: macos-13
             python: '3.9'