fix(cli): bound index discovery scans

[Barry2llen]

Summary

• replace the unbounded App-format metadata scan in leann list with a bounded, pruned directory traversal
• add leann list --max-depth N with a default depth of 5 while preserving direct CLI-format index discovery
• preserve registered App-format projects and full-depth API/HTTP discovery when no CLI depth is supplied
• avoid hiding or double-counting indexes when registered project roots overlap
• add regression coverage and a changelog entry

Testing

• PYTHONPATH=packages/leann-core/src:packages/leann-backend-hnsw python -m pytest tests/test_cli_list_performance.py tests/test_cli_verbosity.py tests/test_ci_minimal.py::test_cli_help -q — 31 passed
• ruff check packages/leann-core/src/leann/cli.py packages/leann-core/src/leann/registry.py tests/test_cli_list_performance.py
• ruff format --check packages/leann-core/src/leann/cli.py packages/leann-core/src/leann/registry.py tests/test_cli_list_performance.py

Fixes #122