Add assign agent option in macro

Editor assessment

Compact and effective: feat matches the change, 'Add' is imperative, the option and its home (macro) are both named, and the header sits well under truncation limits with the PR reference attached. The Co-authored-by trailer is correctly formatted. Two small upgrades would perfect it — a scope like macro (or macros) for changelog grouping, and a body noting what the assign-agent option does at execution time, since macros that reassign conversations affect routing and ownership. For a small feature the brevity is defensible, and the message is easy to scan in any git log.