Generate text file embeddings for semantic cd search

[plumocracy]

Summary

• add FastEmbed/BGE small local embedding generation for indexed text files
• store document chunks and embeddings alongside directory metadata
• show a lightweight search animation for explicit and implied searches
• preserve cd behavior unless the exact local entry exists, then fall back to normal cd semantics

Validation

• cargo fmt --all -- --check
• cargo test --all
• cargo clippy --all-targets --all-features -- -D warnings