docs: separate offline quick start from model setup

[rustfuture]

Add clone/build prerequisites and explain the snapshot and source-reference outputs. Keep offline retrieval in the first command block and move model-assisted answers into an explicitly optional section with setup guidance.

Validation

All listed checks completed locally with exit code 0. Run output paths used temporary directories to preserve committed evidence.

git diff --check
cargo fmt --check
cargo check --locked --all-targets
cargo clippy --locked --all-targets -- -D warnings
cargo test --locked
cargo build --locked
cargo run --locked -- --help
cargo run --locked -- --index evaluation/corpus /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/ri.ri
cargo run --locked -- --semantic evaluation/corpus "index header"
cargo run --locked -- --hybrid evaluation/corpus "index header"
cargo run --locked -- --load-index /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/ri.ri "index header"

Offline commands were exercised; optional Ollama/model calls and Windows were not run.