Stream formatted NDJSON responses incrementally

[ryanfowler]

Summary

• Added a dedicated NDJSON stdout streaming path so formatted NDJSON no longer waits for EOF before rendering.
• Split decoded bytes on newlines, format each record with the JSON-line formatter, and flush after each record.
• Added a unit guard for the NDJSON streaming decision and a PTY integration test that verifies output appears before the stream closes.
• Updated the repo notes in AGENTS.md to reflect the new behavior.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test integration -- --test-threads=1