docs: correct and explain the first task workflow

[rustfuture]

The old provider example reused a cancelled task, referenced a missing workspace and passed the verification command with the wrong separator. Use a fresh queued task, the bundled fixture and semicolon-delimited arguments; add clone/build steps, prerequisites and lifecycle output 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 -- enqueue /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/runtime-data demo-task
cargo run --locked -- status /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/runtime-data
cargo run --locked -- cancel /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/runtime-data demo-task
cargo run --locked -- status /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/runtime-data
cargo run --locked -- enqueue /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/runtime-data coding-task
cargo run --locked -- status /var/folders/v1/5g8zpd5j0h56kyw4dn14f_g40000gn/T/portfolio-quickstart-c03_zqaj/runtime-data

Offline lifecycle was exercised; live model-provider calls were not run. Provider argument parsing and fixture path were checked against source.