*-elevations_histogram.csv (row 2, col 1) gives flight altitude above
ground level for each sequence.  Low AGL (<25-50 m) correlates with
algorithm failures in dense-canopy areas.

Changes:
- find_xml_files: also yields hist_path (*-elevations_histogram.csv)
- parse_agl(): reads AGL from second row, first column (tab-separated)
- agl_m added to sequence metrics dict and TSV output
- --min_agl filter: hide sequences below given AGL from console output
- AGL column shown in console tables
Co-authored-by: Claude <claude@elphel.com>