Document the output model and clarify binary warnings

[ryanfowler]

Summary

• Add a short Output Model section near the README quick start to explain stdout/stderr separation, paging, formatting, and binary safeguards
• Improve binary-response warnings to include the detected content type and exact alternatives
• Update docs and terminal tests to cover the new warning text

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features --lib
• cargo test --all-features --test terminal binary_response
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1