feat: add list memories command

[haosenwang1018]

Summary

• add MemSearch.list_sources() and a CLI memsearch list command for listing indexed memory sources
• provide a small answer to issue Feature Request: List all memories #277 without changing the search/index data model
• add CLI tests for both populated and empty indexes

Testing

• uv run python -m pytest tests/test_search_cli.py -q
• uv run ruff check src/ tests/
• uv run ruff format --check src/ tests/
• git diff --check