egodex: add facade (egodex.py) + runnable demo, switch converter to Hdf5File

Add the thin facade the blog/notebook import (convert -> add_state_features ->
add_skeleton_features -> embed_frames -> query) and a copy-paste egodex_demo.py
showing pose-only, text-only, and combined pose x text queries.

Switch egodex_lerobot.py to read HDF5 via Daft's native Hdf5File type (one row per
episode) instead of the vendored hdf5_example reader. Add requirements-egodex.txt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sgarimel