docs: add a headless first-run example and exit guidance

[rustfuture]

Document build prerequisites and q to exit the dashboard. Add a local JSON/CSV export example for users without an interactive terminal and explain the output paths and platform limits.

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 -- export --json /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/grainx-stats.json --csv /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/grainx-stats.csv

Interactive dashboard opened in a PTY and q exited with code 0; exported JSON decoded and CSV rows were checked. Windows was not run.