Exclude non-microbial host taxa from community embeddings by default

- Add exclude_hosts parameter to CommunityVectorAggregator (default: True)
- Host taxa without embeddings are excluded from coverage calculation
- Enables visualization of host-microbe communities based on microbial members only
- UMAP now includes 77/78 communities (up from 74/78)
- Added --include-hosts/--exclude-hosts CLI flag

This change allows insect symbiosis models (spittlebug, sharpshooter, cicada)
to be visualized based on their bacterial symbionts, excluding non-microbial
host organisms from the embedding representation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: realmarcin