Add indexed search foundation

[plumocracy]

Summary

• add JSON config discovery, SQLite schema, indexing pipeline, and deterministic embedding interface
• add file-content chunk indexing, metadata storage, semantic search routing, and directory type detectors
• add flag-based commands for init, index, search, directory type counts, reset, and shell setup
• add installer support plus integration coverage for search, reset, shell behavior, and install script

Validation

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