feat(demo-got): add semantic search over character bios

[kevinmichaelchen]

Summary

• add bio markdown loader + embedding generation for ingest
• store vector ids on PERSON nodes and add semantic search command
• include biography data files + update demo-got docs
• fix embedding ingest to keep nodes without bios
• clear vector data and indices on reset
• capture helix-recall concept doc

Features

# Ingest with embeddings (vectors stored for search)
cargo run -p demo-got -- ingest

# Skip embeddings for faster iteration
cargo run -p demo-got -- ingest --skip-embeddings

# Clear and re-ingest (now clears vectors/indexes)
cargo run -p demo-got -- ingest --clear

# Semantic search
cargo run -p demo-got -- search "the mad king"

Test plan

• cargo test -p demo-got