GH#5439: Follow the user's threading choice for managed-agent replies

[marcusquinn]

Summary

• keep managed-agent replies top-level when the triggering human message is top-level
• preserve thread-root anchoring when the human writes inside a thread
• retain existing agent-to-agent nesting behavior
• align the base prompt and regression coverage with the user-led threading contract

Verification

• cargo test -p buzz-acp --lib (686 passed)
• cargo test -p buzz-acp queue::tests --lib (112 passed)
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --check
• git diff --check

Resolves #5439